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ANATOMY
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THE KING CRAB
( Limulus polyphemns , Latr.).
RICHARD OWEN, C.B., E.R.S.
y
LONDON:
PRINTED BY TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET.
1873.
ROYAL COLLEGE OF PHYSICIANS
LIBRARY
CLASS
5“«?
ACCN.
SOURCE
7
DATE
TO
THOMAS BELL, F.R.S. Ac. Ac.
This Work, a development of a paper submitted, in fulfilment of his wish, to the
Linnean Society, which prospered under his Presidency, and in which he continues to
take the warmest interest, is affectionately inscribed by his old friend and fellow-labourer,
THE AT.TTHOK.
CONTENTS.
Page
§ 1. Introduction 1
§ 2. External characters 3
§ 3. Muscular System 10
§ 4. Nervous System 15
§ 5. Digestive System !' . . 21
§ 6. Sanguiferous System 23
§ 7. Respiratory System 26
§ 8. Reproduction of Parts 27
• § 9. Generative System 27
§ 10. Development 29
§ 11. Conclusion 3S
§ 12. Description of the Plates 46
i
ON
THE KING CRAB
{Limulus polyphemus, Late.).
§ I. Introduction. — The living representatives of extinct groups of animals have always
had peculiar attractions for my scalpel, especially when the lost group was large and the
dissectible representative rare and exceptional in character. Such, e. g., was the Kivi*,
sole survivor of the race of Moas ; such the Protopterus or Lepidosirenf, the living repre-
sentative of extinct notochordal, protocercal, cycloganoid fishes of palaeozoic seas ; such the
Nautilus, in like relation to the extinct fabricators of chambered and siphonated shells $ ;
such also were Terebratula, Lingula, and Piscina §, as representatives of the Brachiopoda,
and Buplectella, the surviving type in oceanic depths of the fenestrate Ventriculites.
With reference to the singular and interesting palaeozoic Crustacea known chiefly, if
not exclusively, in 1840, as c Trilobites,’ I was for a time uncertain whether to take the
rare Isopod Serolis, of which a specimen was procured for me for that purpose by my
friend Charles Stokes, Esq., E.R.S., the discoverer of the ‘labium’ or lip-plate in Trilo-
bites ( Asaphus platycephalus)\\, or to look for their grade and plan of internal structure
in Limulus.
The authority of W. Sharpe Macleay, after the appearance of his famous “Horae
Entomologicae,” weighed about that time heavily upon us. All who had studied the
Trilobites up to 1843 were of opinion that they were malacostracous. Audouin led the
way by affining them to the Isopoda N ; and Macleay, in. an Appendix to Murchison’s
great work on the Silurian strata, assigned to Trilobites a position as a distinct Order
between the Isopoda and Aspidophora, basing his views on the trilobed character of the
segments in Serolis and Bopyrus, and the character of the eyes in Oymothoa, which “ were
large, sessile, and compound, as in Trilobites. Moreover Cymotlioa and other Isopods,”
he remarked, “ rolled themselves into a hall,” as Trilobites have been found to do before
they perished.
The first general fact or view which influenced my choice in this matter was the
character of the Malacostraca, founded on the number of body-segments, — seven for
* “ On the Anatomy of the Apteryx australis ,” Trans, of Zool. Soc. vols. ii. & iii. (1838).
t Trans. Linn. Soc. vol. xviii. 1839.
+ ‘ Memoir on the Pearly Nautilus {Nautilus pompilius),’ 1832.
§ “ On the Anatomy of the Brachiopoda of Cuvier,” Trans. Zool. Soc. vol. i. (1835) ; also ‘ On the Anatomy of
Terebratula and Lingula,’ Monograph, published by the Palseontographical Society in vol. for 1854. (The subjects
for the anatomy of Disdna, Sow., were referred to the genus Orbicula .)
|| Trans. Geol. Soc. Lond., N. S. vol. i. pi. 27.
5f “ Becherches sur les Bapports Naturels qui existent entre les Trilobites et les Animaux Articules,” Annales des
Sciences Physiques de Bruxelles, tom. viii. (1821).
B
2
the thorax, seven for the abdomen, and, admitting the same number (as indicated by
sense-organs and appendages), seven for the head, = total twenty-one. Now this cha-
racter could not be predicated of the Entomostraca ; some had more, some fewer segments.
Branchipus stagnalis, for example, had eleven thoracic and nine abdominal segments,
besides the head protected by its cephalic shield. In Isaura, in which this shield is also
present and of great size, the number of thoracic and abdominal segments exceeded
twenty-four.
Amongst the Trilobites the part of the body next the shield-shaped cephalic one shows
eight segments in Asaphus platycephcilus, eleven segments in JPhacops, and from thirteen
to fifteen in Calymene, besides an abdomen of eight segments. Then there were departures
in Entomostraca from the Malacostracous numerical or segmental character by defect as
well as by excess, — forms, like Limulus, e. g., with less than twenty-one segments.
Moreover “ the trilobed character of the segments in Serolis and Bopyrus is present
also in Limulus, the segments of its body, markedly in the hinder division, presenting
three elevations or lobes. The eyes, it is true, are large, sessile, and compound in Cymo-
thoa ; but so are the larger pair in Limulus, and more like those of the Trilobite than the
eyes of any Isopods are; the larval Limuli, moreover, roll themselves into a ball”* * * §.
The value of the numerical character of the segments of the body in the question of
the affinity of the Trilobites was pointed out by me in a lecture on Crustacea published
the week after its delivery, April 27th, 1843 f.
Burmeister, whose excellent work on Trilobites appeared at Berlin in a later part of
1843 X, insisted, with equally original views, on the importance of this character ; but,
for his remark that “ Limulus was still more widely removed from the Trilobites than
the Isopods are ” §, I could not see adequate grounds.
All this, however, is now of mere historical interest; and I fully concur with my
experienced colleague, Henry Woodward, Esq., E.G.S., whose labours have shed so
valuable a light on the affinities and homologies of the Crustacea other than those in
which “the normal number of segments is twenty-one,” that “the conclusions of Prof.
Agassiz and James Hall as to the close affinity existing between the Eurypterida and the
Xiphosura are correct ”||. Whether the extension of Dana’s group, Merostomata, as
* Owen, ‘ Lectures on the Comparative Anatomy and Physiology of the Invertebrate Animals,’ ed. 1855,
p. 331.
t In the following terms : — “ The distinction between the Entomostraca and Malacostraca in the number of the
segments of the body is of the first importance in determining the affinities of the ancient extinct Crustacea called
‘ Trilobites.’ ” c Lectures on the Comparative Anatomy and Physiology of the Invertebrate Animals,’ 8vo, 1843,
p. 165.
+ A translation of this work, with notes, by T. Bell & Ed. Forbes, was published by the Ray Society in 1846.
§ In this view, however, Burmeister received the support of Emmerich in Leonhard und Bronn’s Neues Jahrbuch,
1845, part i., translated in ‘ Taylor’s Scientific Memoirs,’ vol. 4, part xiv. p. 253, August 1845. Emmerich defines
the “ Trilobites as a peculiar order, connecting Malacostraca with Entomostraca, but nearer the latter. They are
related to the former by their calcareous crust-like shell, and by their not possessing simple eyes in conjunction with
compound eyes. The Woodlice (Isopoda) have, of all Malacostraca, the greatest resemblance to Trilobites.”
|| ‘ Monograph of the British Fossil Crustacea belonging to the order Merostomata,’ Part I., Palseontographical
Society’s vol. for 1866, p. 9.
3
expanded by Mr. W oodward *, may or may not meet with general acceptance, it is plain
that Limulus has closer affinities Pterygotus and other Eurypteroid Crustacea of palae-
ozoic age than with the Tribolites. My aim, therefore, in finally selecting, in 1843, Limulus
for anatomical research reflecting light on the organization of palaeozoic Crustacea, is
equally fulfilled by the subsequent discoveries of Agassiz f, McCoy j, Hall §, Niesz-
kowski ||, Salter^", Huxley and Woodward**, of extinct Crustacea of as high anti-
quity as the Tribolites, and more closely represented by Limulus. I doubt not therefore
that the following illustrations of the structure of their lingering representative will be
acceptable to Palaeontologists as well as to Comparative Anatomists.
The contributions to the anatomy of Limulus previously made will be noticed in con-
nexion with the sections to which they belong ft.
§ 2. External Characters. — My remarks on this head need be few, and bear mainly on
the intelligibility of the anatomical details.
* ‘ Reports and Proceedings of the British Association, Edinburgh, August 1871.’ Mr. Woodward exemplifies
his views by the following concise parallel : —
“ Order MEROSTOMATA, Dana.
“ Suborder Eurypterida. I. “ Suborder Xiphosura.
“ Ex. Pterygotus (Fossil, extinct).
1. Eyes sessile, compound.
2. Ocelli distinctly seen.
3. All the limbs serving as mouth-organs.
4. Anterior thoracic segments bearing branchiae or
reproductive organs.
5. Other segments destitute of any appendages.
6. Thoracic segments unanchylosed.
7. Abdominal segments free and well-deve-
loped.
8. Metastoma large.
“ Ex. Limulus (Fossil, and living).
1. Eyes sessile, compound.
2. Ocelli distictly seen.
3. All the limbs serving as mouth-organs.
4. All the thoracic segments bearing branchiae or
reproductive organs.
5. Other segments destitute of any appendages.
6. Thoracic segments anchylosed.
7. Abdominal segments anchylosed and rudi-
mentary.
8. Metastoma rudimentary .”
f ‘ Monographie des Poissons Fossiles du Vieux Gres Rouge,’ &c. 4to, p. xix. 1844.
t ‘ Contributions to British Palaeontology,’ Cambridge, 8vo, 1849.
§ Prof. James Hall, LL.D. ‘ Natural History of New York,’ part vi. Palaeontology, vol. iii. 4to, 1859.
j| Archiv fiir die Naturkunde Liv-, Ehst- und Kurlands, erste Ser. vol. ii. 1859.
If “ On some new Crustacea from the Uppermost Silurian Rocks ” (Salter), “ Observations on the Structure and
Affinities of Himantopterus ” (Huxley), Quart. Journ. Geol. Soc. Bond. vol. xii. 1856 ; “ On the Anatomy and Affi-
nities of Pterygotus ” (Huxley & Salter, in Monograh 1, ‘Memoirs of the Geological Survey of the United Kingdom,’
8vo, 1859).
** “ On Eurypterus lanceolatus,” Geol. Mag. vol. i. 1864. British Association Reports, 1864. Quart. Journ. of
the Geol. Soc. Lond. vol. xxi. (1865).
ft- Straus Diirckheim, in his ‘Anatomie Comparee des Araehnides,’ June 1829, pointed out- some particulars of
structure, the “sternum interieur,” e.g., in which Limulus resembled the spiders. This led Latreille to designate the
Limuli “ Crustaces-arachnides ” (Dictionnaire d’Histoire Nat. Art. ‘ Entomologie, Limuli ’). Other resemblances to
Araehnides in organization will be pointed out in the present memoir. Whence I infer that Limulus and the extinct
members of the order Merostomata exemplify a more generalized condition of condylopod organization, from which
the Arachnida, quitting the waters, may have diverged as a special branch of air-breathers.
B 2
4
The body of the American as of the Moluccan King-crab ( Limulus ) consists of three
principal parts — two large, broad, depressed, and shield-shaped, as viewed from above
(Pl. Y a., b), the third long and spike-shaped ( ib . c).
Por the description of the external characters of these parts, which are not here
noticed, I refer to the elementary works on Crustacea and to the undercited excellent
treatise by Van der Hoeven*.
The homologies propounded by the Dutch monographer have not, however, been
generally accepted. “ It is evident,” he remarks, “that the foremost division [c premier
bouclier ’] answers to the head and thorax of insects ; for the feet are attached thereto,
whilst it bears on its upper surface the organs of vision. Thus the head is here con-
founded with the thorax, and we believe ourselves authorized to give to this first buckler
the name of ‘ Cephalothorax,’ which naturalists assign to the first part of the body of
Arachnidans” f.
This homology seems not to have been so evident to subsequent crustaceologists.
Milne-Edwards, Thos. Bell, Spence Bate, Prof. Dana, and, above all, those eminent
observers, Salter, Huxley, Woodward, who have devoted themselves so laboriously and
successfully to the study of the palaeozoic Crustacea, to which Limulus is most closely
allied, reject it. According to them, the * cephalothorax,’ Y. der H., answers only to
the ‘ head ’ of Insects and Crustaceans.
There is, of course, a corresponding discrepancy as to the homology of the second
division of the body of Limulus. “ Le second bouclier repond a l’abdomen des Arach-
nides,” according to V. der Hoeven $. It is the ‘thorax’ of the above-cited later
carcinologists.
At this point I venture to submit the following remarks : — The first division (a in all
the plates), which constitutes, in Limulus , the major part of the entire body, which
includes, besides the mouth, the brain, and organs of sense, also the major part of the
neural axis, the same proportion of the heart and of the genital organs, together with the
stomach, liver, and half of the intestinal canal, has obvious analogies with both head and
abdomen of higher animals. The second division (b in all the plates), which in both
Limulus and Scorpio includes the lamellate respiratory organs, the continuation of the
heart, of the intestine, and of the neural axis, with the terminal outlets of the genital
organs, as obviously repeats characters of both thorax and abdomen of higher animals.
The so-termed ‘ cephalothorax ’ of Arachnology, wrhich is, as Van der Hoeven rightly
recognized, the homologue of the first division of the body of Limulus, does not include
the segments and appendages answering to those called ‘ thoracic ’ in modern crustace-
ology. The ‘ abdomen ’ of Scorpio (Audouin) and of Limulus (Van der Hoeven) does
correspond with the so-called ‘ thorax ’ of carcinologists.
To apply the terms ‘ cephalon,’ ‘ caput,’ or ‘ head,’ to the division of the body of Li-
mulus, above characterized, seems, however, to be an extension of the use of such term
beyond fair and reasonable bounds.
* Yan der Hoeven, ‘ Recherches sur l’Histoire Naturelle et l’Anatomie des Limules,’ fol. 1838. The species which
he dissected was the rapier-tailed Molucca Crab ( Limulus rotundicauda, Latr.).
t lb. p. 10. + Op. cit. p. 11.
5
If like considerations have led careful and conscientious describers to propose definite
terms, giving an escape from wrong analogies suggested by those borrowed from verte-
brate nomenclature, for the various appendages of the exoskeleton of Crustacea* * * §, I venture
to hope that the term * cephaletron ’ may meet with some acceptance as applied to the
anterior division of the body in both Limulus and Arachnids, and that the term * thorac-
etron ’ may have the same fortune in relation to the second division of the body. Both
terms indicate the composite analogies with the three great divisions of the body in
anatomy ; neither of them indicates or infers an homology adverse to the general conclu-
sions which the ablest students of recent and fossil Crustacea have arrived at and agreed
upon.
The Greek term ‘rirpov’ signifies a part of the abdomen; and a part of such cavity is
associated with the ‘ head ’ in the first division of the King-crab’s body, and with the
‘ thorax ’ in the second division. Tor the third division (c in all the plates) I willingly
adopt Mr. Spence Bate’s proposed term of c pleon,’ including therein the part he calls
‘ telson,’ the whole constituting the characteristic ‘ tail-spine ’ of the present singular
genus.
Bor the six pairs of articulate limbs, or appendages, of the £ cephaletron ’ (Pl. II. fig. 2,
ii-vii), I accept the homologies, and consequently adopt the terms applied to them
by Bell f , Woodward +, and others.
These appendages are interesting in the present ancient form of the crustaceous class
through the small amount of differentiation to which they have been subject. The
homologue of the ‘ antennules ’ or ‘ internal antennae ’ (n in all the plates) of higher and
later Crustacea, is a forcipated limb, differing by its less number of joints and smaller
relative size from the succeeding forcipated pairs. It is interesting, also, to note that in
Scorpio, which, like Limulus, goes back to the 4 Coal-measures,’ the corresponding 4 anten-
nules ’ are forcipated. In Limulus, however, the antennules are articulated by c gom-
phosis ’ to the sides of the base of a small c labrum,’ which is wedge-shaped, with the
edge below.
Another analogy to Arachnida is exemplified in the ‘ outer antenna ’ (in, ib.), or second
pair of limbs of Limulus, inasmuch as it is the seat of a sexual character. In the male of
Lwnulus polyphemus it is monodactyle, the last joint being in shape a slightly bent claw
(PI. IV. fig. 1, m). In the male Limulus moluccmus both second and third pairs of
limbs are so modified §. In the females of both species the corresponding limbs are
forcipated (PI. II. fig. 2, in). In both sexes the limbs succeeding the first pair, besides
the addition of two basal segments (ib. fig 3, 1,2), have a marked increase of length, and
go on more gradually lengthening to the sixth (vn, ib.). This pair (PI. II a. fig. 4)
has an additional joint (ib. 7). A long, slender, bi-articulate appendage (ib. r) is articu-
lated to* the outer end of the hind border of the transversely extended haunch (ib. 1).
* As, e.g., “ siagonopod,” “ pereiopod,” “ pleopod,” “ uropod,” &c. proposed by C. Spence Bate in the ‘History
of British Sessile-eyed Crustacea,’ part. i. p. 3 ("October 1861 ). *
f ‘ History of British Stalk-eyed Crustacea,’ p. xx. (1853).
J ‘ Monograph on British Fossil Crustacea,’ &c. 4to, 1866, p. 4.
§ Van der Hoeven, ut suprd, pl. i. fig. 3.
6
The inner border of the haunch is denticulate (ib. p), but in a less degree than in the
four preceding pairs : all are subservient to the preparation of the food for the mouth,
which is surrounded by these carding bases of the cephaletral limbs. A subulate appen-
dage (PL II a. fig. 4, s, & PI. IV. fig. 1, vii. s ) is attached to the inner angle of the
distal end of the fourth joint of the sixth limb. The penultimate joint of the same
limb, instead of supporting merely the opposing blade of the chela or pincer, has four
petal-like appendages (ib. e, t), besides a minute, elongate, slender pincer (ib. u), termi-
nating what seems to be the normal continuation of the limb-joints.
The arrangement of the six pairs of limbs, in relation to the mouth, is shown in
PL II. fig. 2, in a female Limulus polyphemus. It is here seen, as in fig. 1, Pl. IV.,
that these limbs are aggregated in a limited tract of the concave ventral surface of the
cephaletron (a), and are so small in proportion as to suggest their inability to perform
more than an accessory share in the locomotion of the species. The last alone, or
‘ maxilliped ’ (vn), is modified for such function.
Behind this pair of limbs are attached a pair of compressed spinigerous oblong plates
(Pl. II a. fig. 2*, & IV. fig. 1*), slightly divergent, with the spinous border directed
downward or 4 ventrad ; 5 they recall to mind the pectinate appendages of the thoracetron
in Scorpio. In Limulus they close or complete the oral armature posteriorly, form the
‘ 16vre inferieure ’ of Cuvier f, and a corresponding member of the £ trophi,’ according
to Savigny ; but Latreille preferred to regard them as the haunches or jaw-lobes of the
sixth pair of limbs detached The haunch-joints, however, are present, though less
dentated, in the limb vii. (Pl. II a. fig. 5, i) ; but their palpal part (ib. p) is supplied
by nervules having origins distinct from those of the main limb-nerves. The appendages
in question have also their own distinct pair of nerves (Pis. II a. & IV. fig. 1, n*),
arising between the origins of the main nerve of the limb vii. and that of the ganglionic
chord, suggestive of a serial homology with the palpal nervules. I am not, however,
satisfied with this as a ground for regarding the parts in question as detached limb-
palps ; and I, therefore, propose to call them 4 chilaria’ §.
The dorsal surface of the cephaletron is moderately accentuated : certain longitudinal
tracts are depressed or produced inwardly, to afford advantageous attachments to muscles ;
other tracts are elevated to support the eyes, as outlooks, and also bearing defensive
spines.
The ocelli (a 1 in all the figures) are placed one on each side of the anterior elevation
of the mid ridge, which is the highest point toward the fore part of the cephaletron, and
from which the dorsal surface of that broad semilunar shield slopes rapidly down to
the curved digging-edge. The larger compound eyes (a i) are equally favourably placed
for a lateral outlook, each upon the outer part of an elevation about the middle of the
longitudinal lateral ridge, from which the dorsal surface also slopes rapidly to the dig-
ging-edge. The almost horizontal tract between the lateral ridges, bisected by the
median ridge, is traversed by the two longitudinal depressions, which are rather nearer
t “ La levre inferieure est en arriere de la derniere paire de machoires, et formee de deux lames dentelees.” Ta-
bleau elementaire de l’Histoire Naturelle des Animaux, p.452 (8vo, 1797).
X Cuvier’s ‘ Regne Animal,’ ed. 1828, vol. iv.
§ Gr. ^eiXapioy, a small lip.
7
the median than the lateral ridges. These depressions, with the median and lateral eleva-
tions, give the trilobitic character to the carapace or dorsal wall of the cephaletron of
Limulus. Each longitudinal ridge terminates by a short spine posteriorly. They are
much produced in the carboniferous Limuloids, e. g. Prestwichia, as in some Trilobites,
whence they have received the name of ‘ genal,’ or ‘ cheek ’-spines (in Trinucleus , e. g.).
The hind vertical part or border of the dorsal wall of the cephaletron is divided into
a median transverse tract of equal extent with the fore border of the thoracetron there-
with articulated, and two lateral oblique tracts passing outward and backward to the
hinder angles of the cephaletron, which are usually spiked (PL Y, a") . The transverse
posterior tract is vertically thick, as if truncate. At the middle third, included by the
hind deepened ends of the lateral longitudinal dorsal furrows, the upper border rises
like the slopes of a low roof on each side to the median longitudinal dorsal ridge, here
raised into a spine. The border below this is arched for the reception of the fulcra!
levator-process of the thoracetron, which passes beneath the arch. The joints of the
thoracetron with the cephaletron are at the straight lateral tracts on each side the convex
process, entering the arch and at a lower level. Consequently a power tending to pro-
tract or pull forward the process, being opposed by the joints or centres of motion below,
raises the thoracetron upon those centres, and brings it to a line or to a level with the
dorsal plane of the cephaletron, or even raises it to a slight angle therewith. At the
lower part of the transverse hinder tract, outside the piers of the arch, are the pair of
small crescentic pilose depressions (PI. I. fig. 1, h) corresponding with the origins
of the entapophyses within. Erom the posterior spine (ib. k), terminating the lateral
ridge of the dorsal surface of the cephaletron, a lower ridge passes downward and out-
ward, to be lost in the lateral tract of the hind border. The indentation or groove
between the ridge and border is continued along the thick transverse hind part of the
cephaletron, defining the portion below, which articulates with the thoracetron. In this
groove is situated the entapophysial pit ( h ). To the lower part of the so-defined bind
surface of the cephaletron is attached the opercular plate, or first coalesced pair of
lamelliform limbs (Pis. II., fig. 1, IV., fig. 1, vm). I view the groove above described
as a persistent indication of an originally separate segment. This segment, by the
entapophysial pits above and the lamelliform appendages below, belongs to the category
of ‘ thoracetral ’ plates : it is cephaletral only by confluence.
The general aspect of the cephaletron of Limulus reminds one of a rounded spade-
blade, or the blade of a saddler’s knife. It gives forward a digging-edge, curving outward
and backward to nearly twice the breadth of the following segment ; so that this can be
drawn along in the track delved out by the foremost one with least resistance. The hard
chitine (PI. II a. fig. 2, f) of the arched upper surface of the cephaletron ( b ) meets
a flat tract of the same material below (ib. c ), at an acute angle, to form the digging-
edge ( d ) ; and this edge is strengthened by a low ridge, like the carpenter’s e bead,’
running above it, along the extent where most resistance has to be overcome, the { bead ’
subsiding or falling into the edge at the hinder angular spiked ends (Pis. I. and IV. A"),
which terminate outside the thoracetron (ib. B), nearly halfway toward the hind end
of that division of the body.
The flat under surface of the digging-blade (Pis. II., III. c ) is broadest at the point
of most resistance, viz. at the foremost part of the curved edge. From this part the flat
tract extends backward to its hinder border, which forms a pair of bold curves, arching
outward and backward from the hindmost point, which is in the mid line, and in the
form of a retroverted spine, supported by a vertical buttress-like ridge. The under
hard chitine (Pis. II., III. A') rises rapidly from the curved hind borders of the flat part
of the blade toward the softer chitine, forming the arched or vaulted roof and sides
of the cavity concealing the mouth and its environing pairs of jaw-feet as the crab is
viewed from above. Into this vault will slip or be pressed the sand or mud displaced
by the forward and downward thrusts of the spade ; and the burrower will have the
advantage of the additional firmness so given to the cephaletron as a point of resis-
tance to the fulcra and muscular powers then acting from it upon the thoracetron and
the telson, drawing them in, and fixing the latter in the position in which, like an
‘ alpen-stock,’ it can best help forward in the renewed locomotive act, when the muscular
powers and entapophysial fulcra combine their mechanism to again move forward and
press down the great cephaletral spade.
Meanwhile, in the loosened mud or sand so driven back into and filling the under
hollows or vaults, the six pairs of jointed circum-oral appendages are busily at work
sifting the displaced material in quest of whatever organic matter may be included
fit for food.
Save the groove extending along the posterior facet, all traces of the segmental
constitution of the cephaletron are obliterated in its growth, and are recognizable,
externally, only through the appendages and sense-organs of this main division of
the body.
In the thoracetron the segments are indicated not only by the appendages beneath,
but by the pairs of entapophysial pits above, and by the notches and their articulated
spines on each side. These spines are the ‘ epines laterales’ of Van der Hoeven*
(m 1-6, PI. I. fig. 1) ; the fixed spinous productions (ib. n, n ) of the borders of the
alveoli of m 1-6 are termed by Van der Hoeven the ‘teeth’ f. The hindmost of this
series (PI. I. fig. 1, n 7) terminates the lateral border, and projects beyond the posterior
concavity for the articulation of the tail-spine (c).
The trilobitic accentuation of the upper surface of Lvmulus is continued on to the thorac-
etron by the pair of longitudinal depressions beginning where those of the cephaletron end,
and extending about halfway along the thoracetron : in these depressions are the series of
narrow oblong pits, commencing with the pair (ib. li) in the coalesced segment at the back
of the cephaletron, and which, as they indicate the places of attachment of the entapopliyses
projecting from the inner surface, I term * entapophysial :’ there are six in each series
(ib. fig. 2, i 1-i i) in the thoracetron proper, seven with those of the opercular segment, h.
The intermediate rising is subangular, with a spine at the fore part of the ridge, a second
at the part where the longitudinal depressions cease, and a third at the hind end of the
* Op. cit. p. ll.
t “ Nous donnerons le nom de dents aux epines immobiles, et nommerons simplement epines laterales celles qui
sont articulees.” — Ib. p. 11.
9
mid-rising. The lateral, low and broad, convex risings exterior to the depressions
subside where those terminate rather more than halfway toward the hind border of the
thoracetron.
The under surface (Pis. III. & IV. b') defines the cavity lodging the articulate lamelli-
form appendages by a prominent border, within which the chitine loses density, where *
it forms the roof of that cavity. To this roof, or to the ventral surface of the thoracetron,
are attached five large articulate, externally ciliate, lamelliform appendages (ix-xiii in
all the figures), each representing a pair, more or less confluent along the median line.
The similarly shaped appendage (viii in Pis. II A. and IV.) is usually regarded as the
foremost of this series ; it supports the genital outlets, which, are situated on the dorsal
surface of the basal confluent segments (PL IV. figs. 6 & 8, p). It consists of three
joints, of which the third retains the primitive parial distinction, and supports a small
appendage, or fourth joint (ib. fig. 6,4). On the outer or ventral surface two oblique
lines mark off a small median portion of the third segment. On the inner or dorsal
surface the genital outlets are seen at />, and the insertions of the levator muscles at
m u : the articular surfaces at which this coalesced pair have been detached are
marked r.
The succeeding thoracetral appendages are 4-articulate, as is shown in the sections of
ix-xiii in PL II. fig. 1. The basal joints are confluent medianly and ciliate laterally,
like those of the first, they having attached to their upper or dorsal surface, along its
outer two-thirds, the branchial lamellae (Pl. V. fig. 2). The three distal joints preserve
their median distinction : the last joint is narrow, ovate, and projects beyond the
lateral divisions of the broader antecedent joint.
The first pair of confluent lamelliform appendages are commonly termed 4 opercular,’
as they cover the space into which the genital apertures emit the products from the
inner or upper surface of such appendages. But each of the succeeding pairs are equally
‘ opercular,’ inasmuch as they closely overlap each other, shutting in the gills : the
marginal slits, defended by a fringe of cilia, allow the sea-water to filter through to
the branchise, and exclude the particles of sand or mud diffused abundantly, by the
rapid action of the cephaletral limbs, through the respiratory medium during the
burrowing procedures.
The tail-spine (£ pleon ’ and 4 telson,’ c in all the Plates) nearly equals in length the
two antecedent divisions : it is three-sided, with one ridge or angle dorsal and two lateral,
bounding the lower or ventral flattened or slightly excavated surface *. The ridges are
roughened with short retroverted spinules. The base of the tail has three prominences, —
an upper fulcral one, which, in the extended state of the spine, fits into the arched
fossa beneath the back border of the thoracetron : this process receives the insertions of
the 4 levatores muscles.’ The other two prominences form a pair of articular condyles,
adapted to cavities completed below by a pair of prominences of the thoracetron,
developed within the semicircular lower excavation, receiving the condylar part of the
base of the tail-spine;
* The grounds for inferring a confluence of ‘ pleonal’ segments forming the basal part of the spine will be subse-
quently given.
C
10
The structure of the teguments in Limulua polyphemus agrees with that in Limulus
moluccanus *.
§ 3. Muscular System. — The parts sent inward from the crust or exoskeleton are those
that afford attachment to muscles, and those which also form or contribute to the joints
of the articulate appendages. They are termed ‘ entapophyses * and * apodemes.’ The
c apodemes ’ that relate to the cephaletral limbs (PI. V. ii-yi) are broader and more
complex than those of the thoracetron ( ib . yii-xiii). The most conspicuous entapophyses
are the following : — A pair of oblong lamelliform processes descend from the segment
confluent with and forming part of the hind border of the cephaletron at the parts
indicated by the ciliate depressions (PI. I. fig. 1, h ). Six pairs of similar, but rather
smaller, processes project into the cavity of the thoracetron, from the inner surface of
the parts indicated by the oblong depressions (PI. I. figs. 1 and 2, i 1-6). These serve
to give attachment to and augment the force of muscles. Analogous entapophyses are
developed in most of the articulations of the limbs (PI. II a. fig. 3, c, e, g) for a like
purpose. All these internal processes assume more or less of a cartilaginous character,
losing the hardness and colour of the outer crust as they extend inwards.
The main movements of Limulus in locomotion are those of inflection and extension of
the cephaletron upon the thoracetron, and of the tail-spine upon the latter, and
reciprocally.
The fixed points from which cephaletral muscles act upon the thoracetron are afforded
not only by the apodemata and entapophyses, but also by the representative of an internal
skeleton. This (PI. II a. figs. 1 and 2, h ) is situated partly in the angle between the
gullet and stomach, thence extending backward a short way along the interval be-
tween the beginning of the intestine and the neural axis. It is an oblong sub-
quadrate plate of sclerous or fibro-cartilaginous tissue, and is chiefly related to the
attachment of muscles (PL IY. fig. 6). It was likened by its discoverer, Straus Durck-
heim, to an internal cartilaginous sternum, and may answer to the part which he so
terms in Arachnida. I shall refer to it, without any wider homological signification, as
the * entosternon.’
Levatores thoracetri. — The extensors or, more properly, ‘ levators ’ of the thoracetron
are a pair of powerful muscles, the fibres of which rise from the low inner ridges
indicated or formed by the longitudinal medilateral grooves or inflections of the
carapace f. This feature in the accentuation of the upper crust of the cephaletron
relates to such favourable condition of origin of the * levatores thoracetri.’ The pair
come into contact at the median line, filling the hollow of the roof, of which that line is
the mid ridge : their longitudinal fibres (PI. II a. fig. 1, m i) intervene between it and
the pericardium, as they pass backward to be inserted into the anterior and upper
transversely convex process of the thoracetron, g, which enters the corresponding arch,/,
of the cephaletron.
Depressores thoracetri. — The flexors or ‘ depressors ’ of the thoracetron rise from the
* Van der Hoeven, ut suprd, p. 15.
t The corresponding grooves rendering Asaphus &c. * trilobitic ’ most probably indicate analogous ridges or
entapophyses for the flexor muscles of the segments.
11
dorsal surface of the hinder third of the entosternum (Pl. IV. fig. 5, m 2), divide as they
pass backward into two groups, or a pair, the fibres of which ascend obliquely on each
side the intestine, and subdivide into fasciculi (PI. I. fig. 1, t, t), to be inserted into the
entapophyses of the thoracetron.
Prvetrahentes entosterni. — The power of the 4 entosternon ’ as a fixed point or fulcrum
is provided for by other muscles. A strong longitudinal subdepressed fasciculus rises
from the inner surface of the fore part of the cephaletron on each side, the fibres of which
slightly converge as they pass backward to he inserted into the anterior angles of the
entosternon (PI. IV. fig. 5, m 3) *. They tend to draw that part forward, and resist
the backward displacement of it, which would otherwise ensue in the action of the
* depressores thoracetri ’ (m 2). The ‘ prsetrahentes entosterni 5 are the main origins or
‘ fixed points,’ functionally, of the great muscles, made £ digastric ’ by the intervention
of the entosternal fibro-cartilage, which depress the thoracetron : when the insertional
lamellse of the * depressores thoracetri ’ become fixed points, they act through the medium
of the entosternon as origins of the digastric muscles deflecting the cephaletron. In like
manner, when the insertions of the ‘levatores thoracetri’ become the fixed points or origins,
those muscles will oppose the 4 depressores cephaletri,’ and become 4 levatores ’ of
that part.
Levatores antiei sterni. — But the singular structure which acts functionally as
4 endoskeleton ’ in Limulus has additional powers given to it by muscles which, like the
mainstays of a mast, steady it in the transverse or lateral directions. Prom near the
fore part of the dorsal surface of the entosternon diverge a pair of sclerous processes,
which become tendons of a pair of muscles (PI. IV. fig. 5, m 4)f , about half the size of
the 4 protractores ’ ( ib . fig. 5, m 3) and which have their fixed points in the antero-lateral
parts of the cephaletron. The 4 levatores,’ by their direction, tend to raise and draw for-
ward the entosternon, and so add their power to the protractors when these muscles are
made to act in combination with the 4 depressores thoracetri ;’ but to the degree in which
their oblique course would tend, if one of the pair acted singly, to pull the entosternon
sideways, their combined action would add to its fulcral power in relation to the move-
ments of the two chief divisions of the body.
Levatores laterales entosterni. — The steadying of the entosternon is more directly
attained by a series of fibres which, rising from the ridges due to the inflection of the
lateral longitudinal grooves of the cephaletron, descend and converge to be inserted into
the posterior half of the lateral borders of the entosternon (ib. m 5).
Levatores postici entosterni. — Pasciculi from the dorsal surface (PI. IV. fig. 5, me),
which seem to be the fore part of the series of 4 depressores thoracetri,’ ascend, as
they retrograde, to be inserted into the lamelliform entapophyses rising from the hind
border of the cephaletron, which seem to initiate anteriorly the series of shorter and
smaller ones descending from the thoracetron. With the insertion, or rather origin,
of the above entosternal muscles, their action would be to retract and raise the
entosternon.
The functions of these 4 levatores entosterni,’ in relation to the fixation of the endo-
* Van derHoeven, op. cit. p. 47, pl. iii. fig. 7, b. + Ibid. fig. 7.
c 2
12
skeleton, are more especially in opposition to muscles arising from its lower and lateral
parts to be inserted into basal entapophyses of the five posterior pairs of cephaletral
limbs. But the principal muscles acting on the basal joints of these members arise
from the ‘ apodemata,’ or inflections of the ventral crust (PI. V. fig. 1, ii-vi), forming,
or rising from, the articular cavities in the cephaletral plastron for those limb-
segments *.
The ‘ levatores telsi’ (PL I. fig. 2, u, u ), if I may be permitted to latinize Spence
Bate’s term for the ‘ tail-spine,’ rise from the upper median lateral parts of the inner
surface of the thoracetron; the median fasciculi form one elongated muscle (PI. II a.
fig. 1, m 7), which is inserted into the upper basal process, and directly tends to raise
the spine : there are two shorter lateral masses ( ib . m e) converging to be inserted into
the same process, but which, if acting independently, would draw the spine outward as
well as upward. Both median and lateral muscles acting together would raise the spine
forcibly, or if the spine were the fixed point, and the thoracetron depressed at an angle
therewith, would tend to raise that part.
Depressores telsi. — Two shorter and broader but powerful muscles {ib. m 9), having
the double oblique or penniform disposition of fibres, rise from the lower terminal part
or segment of the thoracetron {ib.), and converge to be inserted into the sides of the
basal entapophyses from below the articular condyles of the tail-spine. These, combining
in action, depress the tail-spine ; their lateral portion, combining with the corresponding
one of the levator telsi, draws the spine to that side. When the spine was fixed the
muscles would act as flexors, extensors, or abductors of the thoracetron.
Muscular fasciculi for the protraction and retraction of the thoracetral appendages
rise from the apodemata of that division of the body.
Frotractores branchipedum. — The limb-plates of each lateral moiety of the broad gill-
bearing lamella has two principal muscles : one, arising from the outer part of the
apodeme in advance, subdivides into fasciculi, which descend, penetrating the fore or
under surface of the gill-limb (PL II a. fig 1, m 10), and radiate therein to be attached
to the several segments. These fibres protract the limb, change its recumbent for the
erect position, and in that movement separate the gill-plates and facilitate the flow of
water through their interspaces.
Fetractores branchipedum. — These muscles rise from the base of the apodeme of their
own gill-foot, near the place of articulation of the latter, and spread upon the hinder,
inner, or upper surface of the proximal lamelliform joint before penetrating the interior
of the succeeding ones. They retract or draw up the gill-feet, approximate and press
together the gill-plates, and squeeze out the water from their interspaces. The inser-
tional fibres of this muscle are shown on the anterior thoracetral lamelliform limb, which
serves as a cover or c operculum ’ to the genital outlets (Pl. IV. fig. 6, m 11).
Some small fasciculi, combining their insertions with the proper muscles of the
branchipeds, have attachments to the thoracetral entapophyses, and seem to combine a
levator action upon the branchipeds with that of the * depressores thoracetri.’
In the cephaletral limbs (m-vi, Pis. II. and III.) the haunch-joint {coxa, 1) is of great
* Van der Hoeven, op. cit. pl. iii. fig. 10, b, e.
13
transverse extent, and besides affording insertion to the apodemal muscles, which forcibly
work the carding-plate, or ‘ palpus 5 (PI. II a. figs. 2-5, p), gives origin to muscles acting
on the second joint or ‘ basis 5 ( ib . ib. 2).
Extensor basis pedis. — One of these, of small size, is an extensor of the ‘ basis 5
(PI. II A. fig. 5, a), but only in a slight degree.
The ‘ flexor basis ’ {ib. b) is a larger, especially broader, muscle, and bends the ‘ basis ’
forcibly upon the £ coxa 5 or first joint.
Flexor merii cnemiique. — The muscle {ib. d ) arising from the * basis 5 (2) is supple-
mented by other fibres from the c merion,’ or third joint (3), proceeding penniform-wise to
an entapophysis, c, attached to the base of the £ cnemion,’ or fourth joint (4). The action
of this series of fibres is to bend both merion and cnemion. An £ extensor of the cnemion ’
is feebly developed.
Flexor propedis. — The cavity of the cnemion is chiefly occupied by the penniform
flexor, f of the ‘propes’ (5), upon which it acts chiefly through the medium of the
£ apodeme,’ e, attached to the base of that joint.
Flexor dactyli. — In like manner the penniform muscle, in the swollen basis of the
propes, draws, through the medium of the entapophysis, g, the dactylus, e, powerfully, in
contact with the claw-like process of the propes, 5.
In the propes, or fifth joint, of the maxilliped, fasciculi of the muscular fibres are
grouped to be inserted into the short basal apophyses of the lamelliform appendages
(PL II a. fig. 4, e, t ), which they tend to approximate, or to close upon or around
the terminal chela {ib. 7). These plates, which radiate from the end of the £ propes ’
like the petals of a flower, are expanded by being pressed against the mud or sand,
and seem to require muscles only for closing them, so as to facilitate the withdrawal
of the limb. The application of the maxillipeds in locomotion was observed by W. A.
Lloyd, Esq., the constructor of the Aquarium at Hamburgh, of which he was for some
years the conservator, and subsequently the constructor of that at the Crystal Palace, of
which Aquarium he is now the manager.
At Hamburgh, specimens of Limuli were kept alive from the year 1865 to 1870. Prom
his observation of these Mr. Lloyd informs me, ££ The ulterior pair of limbs ” (maxillipeds,
vn) ££ are not employed for walking, but exclusively for burrowing. These limbs are
terminated by four long stiff lobes of an oval or leaf-shape, jointed at the base, on the leg,
and capable of being opened and closed in a four-radiate manner. When it wishes to
burrow, these two limbs are, sometimes alternately and sometimes simultaneously, thrust
backwards below the carapace, quite beyond the hinder edge of the shell ; and in the
act of thrusting, the lobes or plates on each leg encounter the sand, the resistance or
pressure of which causes them to open and fill with the sand,, a load of which at every
thrusting operation is pushed away from under the crab, and deposited outside the
carapace. The four plates then close, and are withdrawn closed, previously to being
opened and charged with another load of sand ; and at the deposit of every load the whole
animal sinks deeper into its bed, till it is hidden all except the eyes. The great hiding-
shield of a carapace again prevents one from seeing whether this excavating work is aided
by the fanning motion of the abdominal false feet, as is the case with the British Lobster ;
14
but I think there is such fanning, as I have seen signs of sand being driven through the
sand-orifices as if urged by a current of water.
“ The tail-spine of Limulus is used in locomotion in the following manner : — The animal
having climbed up a rock in the Aquarium till it has got near to the top of a tank (which in
Hamburgh contained thirty inches of water in depth perpendicularly), and having assumed
a vertical position, leaves go its hold on the rock, and allows itself to fall backwards ; but
its downfall is instantly checked, and the creature propelled upwards by a downward
flap of all the strong overlapping false feet ; and when the impetus given by them has
ceased, the animal sinks down, but is prevented from falling prone on the floor of the
tank by alighting on the tip of the perpendicularly hanging-down spine. The moment
that is done, and before the creature has lost its balance on the spine, the false feet make
another flap, and give another impulse upwards and forwards ; and so it progresses by a
combination of swimming and hopping, or by a succession of slow hops on one leg, as it
were ; and all this time the position of the carapace is slanting, the top of the carapace
inclining downwards at an angle of about 45°, the second segment of the body beiug
at another inclination, and the tail-spine hanging freely vertically, as before mentioned ;
and by being brought down by its joint at various deviations from the upright one, the
spine changes the direction of the march, while the false (swimming) feet effect the actual
propulsion.
“ The Limulus was fond of thus going about at night (generally remaining on the
sand all day). Another use was made of the tail-spine, as a lever by means of which it
righted itself when it fell off a rock on its back. The spine is then bent ; i. e. its
point is planted in the sand so that it makes an acute angle with the carapace, which is
then so far raised that some of the feet are enabled to grasp a projecting surface, either
longitudinal or vertical, or at some combination of the two ; and the crab then turns
over.”
The maxillipeds, no doubt, aid in burrowing, as observed by Mr. Lloyd ; but the chief
fossorial agent, as indicated by the size and disposition of the principal muscular masses,
is the cephaletral digging-shield.
The operation of this is described in the subjoined note on the locomotion of Limulus
polyphemus as observed by the Lev. S. Lockwood, Ph.D., in its native haunts (Lariton
Bay, New Jersey, U. S.).
“ The King-crab delights in moderately deep water, say from two to six fathoms. It
is emphatically a burrowing animal, living literally in the mud, into which it scoops or
gouges its way with great facility. In the burrowing operation the forward edge of the
anterior shield is pressed downward and shoved forward, the two shields being inflected,
and the sharp point of the tail presenting the fulcrum as it pierces the mud, while under-
neath the feet are incessantly active, scratching up and pushing out the earth on both
sides. There is a singular economy of force in this excavating action ; for the alternate
doubling up or inflecting and straightening out of the two carapaces, with the pushing-
purchase exerted by the tail, accomplish both digging and subterranean progression.
Hence the King-crab is worthy to be called the c Marine Mole 5 ” *.
* ‘ The American Naturalist,’ 8vo, vol. iv. 1870, p. 257.
15
§ 4. Nervous System. — The chief part of the neural axis is in the form of an elliptic ring
girting the oesophagus. Of this ring three views are given — one from above (PL Y.
fig. 1, a, b), one from below (PI. IV. fig. 1, a, (3), and one from the side (PL II a.
fig. 1, a, (3), in order to show, besides the shape of the part itself, the precise position of
the nerves arising therefrom or connected therewith. Por the origin of a nerve is an
important element in determining the homology of the part it supplies ; and such de-
terminations have weight, as will be seen, in wider questions, extending, in the case of
Limulus, e. g., even to class-affinity.
The part of the neural axis anterior to the oesophagus (Pis. II., III., Y. a), and which,
were the tube straightened and the mouth brought to its ordinary position at the fore
part of the body, would be superior or dorsal in position, is an oblong mass, concave
where applied to the tube (Pl. Y.), convex on the opposite side (Pl. III.), 3 lines in
length, and 4 lines in posterior or basal breadth, where its angles are continued into
the side parts of the ring (Pl. II A. fig. 1, (3). There is no trace of lateral bipartition
of the supercesophageal or cerebral part of the neural axis. The substance of the ring
shows the same axial or longitudinal extent behind as before the oesophagus ; viewed
from below, as in Plate III. fig. 1, it seems to extend rather further before contracting
to form the ganglionic chord. The narrowest parts of the ring are at the sides of the
oesophagus ; but this is transversely ; vertically the substance there is equal to that of
the hind part of the ring (Pl. II a. fig. 1, (3). Two commissural hands unite the lateral
parts of the ring (Pl. Y.). The ganglions (Pis. II a., IV., S, e, l, v, 0) are confined to the
thoracetral region.
Ocellar Nerve, n a. — The first pair of nerves is the ‘ ocellar ’ (Pis. II., II a., III., IV.,
V., n a). They rise, with an interval of their own diameter, from the fore and upper part
of the brain (Pl. Y. a), diverge with a gentle curve as they advance, bend round the front
convexity of the stomach (Pl. II a. fig. 1, n a), and ascend, converging to terminate each
in its ocellus (ib. a 1). The length of the nerve is two inches.
Ocular Nerve, n A. — The second pair of nerves is the ‘ocular’ (ih. jia). Each nerve
rises from a small conical process of the brain (Pl. V.), where the special quality of an
optic ganglion may be surmised to dwell. An interval of the basal breadth of the
swelling divides the ocular from the ocellar nerve. The ocular nerve curves upward,
crosses the gizzard near the pylorus (Pl. II.), then more abruptly bends outward (Pl. V.),
coasting round the apodeme (n) of the second limb, and retrograding obliquely to the
compound eye, near which the nerve divides into a larger dorsal and smaller ventral
chord. The dorsal division (Pl. IY. fig. 2, d) soon expands, and resolves itself into a
fasciculus of nervules, which subdivide, and finally supply or form the retinae of the
lenses at the upper and hinder part of the compound eye. The ventral, which is also the
anterior division (ib. v), is continued further before expanding and resolving into the
plexus which supplies the retinae to the lenses at the lower and fore part of the com-
pound eye.
Gastric Nerves. — Two pairs of nerves arise posterior to the ocular pair. The nerves of
the first pair (n 3, Pl. Y.) pass forward, give filaments to the oesophagus and stomach, and
are continued on into the hepatic and ovarian substance at the fore part of the carapace.
16
First Epimeral Nerve. — The nerves of the second pair (on the outer side of the
symbol n 4, PL V.) diverge, extend along the fore part of the anterior low and simple
apodeme (n), subdivide, and are lost in the tissues of that part of the segment answering,
in position, to the epimerals * of the type segment. The term ‘ epimeral ’ will, accord-
ingly, be given to this and the succeeding serially homologous nerves.
Second Epimeral Nerve. — A somewhat larger nerve rises immediately behind the
foregoing; the pair (PL V. n 5) slightly diverge and ascend, when each second epimeral
nerve bifurcates. One branch coasts along the anterior apodeme, the other inclines
toward the interspace between that and the second more prominent apodeme, supplies
muscular fibres thence arising, and gives off a recurrent filament (n 6), traceable along
the outer ends of the succeeding apodemes (ii-vi, Pl. V.), at a little distance from them,
as far as the seventh epimeral nerve, where this begins to ramify.
Antennular Nerve. — This nerve (n n, Pis. II., II a., III., IV.) rises from the under
part of the base of the brain (« fig. 1, Pl. IV.), is rather larger than the second epimeral
nerve, and supplies the first small forcipated pair of limbs — the homologue of the first or
inner pair of antennse in higher and more differentiated Crustaceans.
Two filamentary nerves rise from the interval between the antennular and antennal
nerves at the beginning of the lateral parts of the ring (shown in Pl. IV.).
Antennal Nerve. — This nerve (n m) is more than twice the size of the antennular
one ; it supplies the second limb (Pl. IV. in), which is sexually modified in the male
Limulus polyphemus. This limb, forcipated like the rest in the female, is the homo-
logue of the outer and larger pair of antennse in higher Crustaceans ; and its origin is
prse- or super-oesophageal in Limulus.
Third Epimeral Nerve. — Prom the dorsal aspect of the lateral part of the neural ring
rises the third ‘ epimeral nerve ’ (n 7, Pl. V.). It runs forward and outward above
the interspace between the second (ib. n) and third (in) apodemes, and is resolved into a
plexus of filaments beyond that interspace, which are lost in the glandular and other
tissues of that region.
Mandibular Nerve. — The third limb-nerve (Pl. IV. n iv), of the same size as the
second, comes off behind it, from the lateral part of the ring, (3, and supplies the limb
homologous with the * mandible ’ (so called) in higher Crustaceans ; which limb is marked
iv in Plates II. & III., where the nerve is traced through the coxal and basial joints.
Fourth Epimeral Nerve. — Prom the dorsal aspect of the corresponding part of the
neural ring is sent off the ‘ fourth epimeral nerve 5 (n 8, Pl. V.) having the same course
and apodemal relations as the second and third of this series.
Premaxillary Nerve. — This, with a more posterior origin than the mandibular nerve,
repeats the characters of that nerve, in relation to the fourth limb, or homologue of the
‘ praemaxilla,’ or first or anterior maxilla, in higher Crustaceans. The nerve {n v) is
shown in Plates II. & III., entering and traversing the * coxa 5 and ‘ basis’ of the limb
marked V. This limb is sexually modified in the male of Limulus moluccanus , but not
in the species here dissected.
Fifth Epimeral Nerve (n 9, PL V.). — This repeats the relative position of origin to
* Owen, ‘ Lectures on Invertebrata,’ p. 298.
17
the premaxillary nerve which the antecedent one (n 8) bears to the mandibular nerve,
and holds the same relation in its course to the apodemes iv & v. It forms the plexus
beneath that of the optic nerve, beyond which the filaments are lost in the tissues there.
The optic nerve crosses dorsad of the first four epimeral nerves in its course to the
compound eye.
Postmaxillary Nerve. — The fifth limb-nerve {n vi, Pis. II a. & IV.), with a more pos-
terior origin and a course more obliquely backward, repeats in limb vi — the homologue
of the postmaxilla, or second maxilla, in higher Crustaceans — the characters of the ‘ pre-
maxillary nerve ’ in relation to limb v.
Sixth Epimeral Nerve (n 10, PI. V.). — This repeats the same relative position of
origin to its answering limb-nerve as does n 9 ; it is continued further obliquely back-
ward before bending outward to its interapodemal space, and bifurcates before entering
there, the hinder division descending to supply the strong adductor muscle of the sixth
limb (vii in Pis. II a. & IV.).
Maxillipedal Nerve {n vii, Pis. II a. & IV.). — The nerve supplying that limb has
its origin between the postmaxillary nerve and the beginning of the abdominal gan-
glionic chord ~y. It repeats the character of the antecedent limb-nerves in relation
to its own articulated appendage, which is the homologue of the 4 maxilliped ’ in higher
Crustaceans.
Palpal Nerves. — At the interspace between the origins of the postmaxillary and maxil-
lipedal nerves, as in that between the latter and the ganglionic continuation of the neural
centre, arise filaments which supply the spinigerous process or 4 palp ’ of the compressed
denticulate haunch-joint (PI. II A. figs. 2 & 5, p).
Chilarian Nerve (Pis. II A. n x, & IV. n*). This rises between the origins of n vii
& n viii ; it is appropriated to and richly ramified in the leaf-like spinigerous appen-
dage, articulated behind the base of the maxilliped, and closing posteriorly the circumoral
armature. The serial homology of the chilarian with the palpal nervules lends some
countenance to that of the appendage, so supplied, being a detached spinigerous process
or c palpus ’ of vii.
Seventh Epimeral Nerve ( n 11, PL V.). — This arises dorsad of the origin of the max-
illepedal nerve, passes backward and outward to its proper apodeme (vi), where it
divides, and, running onward, ramifies to supply the tissues in the hinder produced angles
of the cephaletron.
Eighth Epimeral Nerve {n 12, PL V.). — This is one of the same system of dorsal nerves,
succeeding the seventh ; it passes backward and slightly outward along the dorsal margin
of the seventh apodeme (vii), and dips into the articular depression between the cephal-
etron and thoracetron.
Opercular Nerve. — The hindmost pair of cephaletral ventral nerves (n, Pl. II A. &
n viii, Pl. IV.) is given off at or just before f the continuation of the neural ring into
the 4 ganglionic chord.’ The nerves of this pair run along the sides of the latter for
about 8 lines, then slightly diverge, curve outward, and send off one or two filaments
laterally, before descending to penetrate the base of the anterior or opercular leaf-foot,
t This lends countenance to the idea that the ‘ opercular limb,’ viii, is the last of the cephaletral series.
D
18
or coalesced pair of limb-appendages of the segment anchylosed to the hack of the
cephaletron.
The corresponding dorsal pair of nerves. ( n 13, PI. V.) also course along the sides
of the ganglionic chord before diverging to ramify in the middle third of the interspace
between the cephaletron and thoracetron.
The ganglionic chord (7, Pis. II a., IV.) extends backward about an inch before its
first ganglion (8) is formed ; this is followed by three others (e, £, v) and a terminal
swelling (0) situated about an. inch and a half from the joint of the tail-spine. The
interganglionic tracts average in length about lines, slightly shortening as the chord
recedes.
Each of the four anterior ganglions gives off two pairs of principal nerves, one dorsal
and anterior ( n 14-17, PI. V.), the other ventral or posterior (n ix-xii, Pis. II a. & IV.),
these terms not being absolute, but meaning the approximate relative position of the
places of union of the nerves with the ganglion. The dorsal nerve is a serial repetition
of the antecedent epimeral ones. It passes outward along the contiguous apodemal in-
terspace, on emerging from which it divides ; the posterior branch quickly subdivides ;
the anterior branch continues further before subdividing ; all these filaments incline ob-
liquely backward before distributing themselves among the tissues of their corresponding
abdominal segments. The dorsal nerves (n 17, PI. V.) from the fourth ganglion run
backward a short way before inclining outward. A filament of each of the thoracetral
epimeral nerves can be traced to the movable side-spine of its segment.
The ventral nerve goes obliquely outward and backward to the space or joint between
its own segment and the one in advance, penetrates the branchial leaf-limb at the part
or moiety of its own side, distributes many filaments to the basal joint, and is continued
on through the second and third joints, before being finally resolved in the fourth and
terminal joint (PI. II a. n ix-xii). More minute filaments are sent off, usually between
the origins of the two chief nerves, from the four ganglions.
The terminal ganglion (0, Pis. II a. & IV.) represents the coalescence of the nerve-
centres of at least three segments. Its anterior dorsal pair of nerves (n 18, PI. V.)
traverse the interval between the fifth (xn) and sixth (xm) thoracetral apodemes, in a
course more obliquely backward than the antecedent pairs. The corresponding anterior
ventral nerves (n xm, Pis. II a. & IV.) supply the sixth pair of leaf-limbs or fifth
branchial pair (xm). Below the origin of this pair, nervous filaments (r, PI. II a.) pass
off to the lower fourth of the intestine.
The second ventral pair {n xiv, ib.) is chiefly distributed to the fibres of the flexor
muscles of the tail-spine, arising from and occupying the soft, rather tumid tract, which
resembles a leaf-foot soldered down to form the covering of the hindmost part of the
ventral surface of the thoracetron.
The third pair of principal nerves from the terminal ganglion represents a bifid con-
tinuation of the neural axis (PI. IV., 1). After a course of about three lines, each
sends off a nerve ( n xv, PI. IV.) belonging to the ventral series, which supplies the
hindmost or postanal region of the abdomen affording the articular surface for the tail-
spine.
19
After sending off the above nerve, each continuation of the chord forms an oblong loop
( ib . k), which, prior to the removal of the vascular sheath, looks like a ganglionic swell-
ing* ; beyond which the chord (A, Pis. II A., IV.) continues along the side of the tail-
joint, and, on entering the cavity of the tail-spine (c), resolves itself into a fasciculus
of fine nerves (ih. pi), resembling the ‘ cauda equina ’ of anthropotomy. But in this
bundle a principal filament, or continuation of the chord (PI. II a. p), can he traced
about a third of the way down the spine. These nerves seem to constitute the major
part of the tissues in the hollow of the spine, and render a marvellous supply of neurine
to so hard, inflexible, and seemingly insensible a part.
Each chord, A, from the ganglionic loop sends off nine nerves, four directed toward
the ventral (PI. IV. fig. 1, pi), four towards the dorsal (PI. II a. fig. 1, a 1-4) region of the
spine : the ninth nerve being of larger size, claims to he the continuation of the bifid
neural axis. If the dorsal and ventral divisions he regarded as those of four nerves
serially homologous with such divisions of antecedent primary pairs, they would indicate
as many segments coalesced in the fore part of the spine. The ninth nerve and its divi-
sions supply in a similar way the rest of the tail-spine.
Are the phenomena of this terminal part of the nervous system of Lvmulus devoid of ho-
mological significance P It seems to he otherwise, for any thing that one can see needing
such supply of nerves in the interior of the hollow spine. All, however, that embryology
has yet shown of the development of this part is, that in the interval between exclusion
and the first moult it buds out of the posterior part of the thoracetron, does not shrink
up to it, and only feeble or doubtful traces of a segmentation have been noticed in the
embryonal but late-growing ‘pleon.’ Nerve precedes crust in hlastemal differentiation :
the earlier tissue obeys the type, the later tissue the adaptive departure therefrom. A
superficial glance catches the result as a ‘ spinous process ; ’ deeper insight discerns the
body-joints masked by the outer connation. Neither development, rightly understood,
nor adult structure gives any countenance to the notion that the tail-spine of Lwiulus
is a mere process or appendage growing from the dorsal part only of the terminal segment
of the thoracetron.
Seeing the relations of the pleonal nerves as continuators of the neural axis, and the
like relation of the artery of the spine to the dorsal vessel, I long ago concluded the spine
itself to he a continuation of the series of body-segments, to be serially homologous
therewith, and not with their ‘ appendages.’ The coccygeal style of the frog’s endo-
skeleton f is analogous to the tail-spine of the King-crab’s exoskeleton. The antecedent
part of the thoracetron (b") wherewith the spine is articulated has no limbs. Is it also
part of the pleon ? and does the postganglionic part of the neural axis indicate the
extent of such part ?
In anatomizing, in 1843, my first-received specimens of Limulus, the details of the
nervous system were followed out by my then anatomical assistant, Mr. Henry Goadby,
and well exemplify his peculiar skill and patience.
The nearest approaches to the type of nervous system above described we found to be,
* As represented by Van der Hoeven, op. cit. pi. iii. fig. 2 c.
t Owen, ‘ Anatomy of Vertebrates,’ vol. i. p. 49, fig. 44 c.
D 2
20
not in the Crustacea most resembling Limulus in general shape and proportions (the
Brachyures, e. g.), hut in Arachnids and Myriopods.
In a Scorpion (Buthus africanus) the two cerebral or superoesophageal lobes are fused
together, send off nerves to the chelicera, or antennal homologues of the limbs n in
Limulus , and to the eyes ; they are connected behind with the stomatogastric nerves,
and laterally, by means of short and thick ‘ crura,’ with the suboesophageal mass. This,
as in Limulus, represents the ganglionic centres of several pairs of nerves, including
those of the chelate palps, the homologues of which come off in Limulus, as in other
Crustacea, from the superoesophageal mass. The four pairs of cephaletral limbs suc-
ceeding the, c palpi ’ in Scorpio, also derive their nerve-supply from the suboesophageal
part of the annular centre. Prom this are continued the ventral chords along the thorac-
etron, successively developing seven closely fused pairs of ganglia ; the terminal chords
distribute, in the pleon, or jointed tail, pairs of nerves to the right and left, and are
traceable to the bent and perforated tail-spine, representing a ‘ telson,’ but forming, by a
mysterious modification, the poisonous weapon of the Scorpion.
The nervous system of this c air-breathing Merostome ’ has been well figured by New-
port * ; and I supply additional illustrations of that of a Myriopod f, which may repre-
sent a condition of the nervous system in the Trilobites.
The superoesophageal or cephalic portion of the neural axis in Julus terrestris (Plate
II. figs. 6, 7, 8) is more transversely extended than in Limulus, and is less obscurely
divided into the right and left ganglions or side-lobes, of which the upper and outer ends
are produced as short and thick optic lobes, which are resolved halfway toward the
compound eyes into a plexus of filaments, distributed, as in Limulus, to the component
ocelli. Prom the fore and outer part of each cephalic lobe two distinct nerves proceed
to the short 7-jointed antenna of its side ; below these a pair of nerves proceed to the
palpless mandibles (fig. 6). Prom the hind and underparts of the cephalic lobes the
thick continuations of the neural axis descend and girt the gullet, beneath which they
become resolved into two chords or rings : the anterior one anastomoses with its fellow,
forming a simple ring ; the posterior and larger chords converge at an acute angle, to
be continued into the ventral body-chord, which shows little of a ganglionic structure.
The converging head-chords are, however, united by a transverse commissure before
retrograding to the ventral body-chord. This commissure and the anterior ring recall
the filamentary cerebral commissures in Limulus (Plate V.) ; the anterior oesophageal or
pharyngeal ring in Julus is also homologous with that shown by Lyonnet in the larva of
Cossus ligniperda.
The stomato-gastric system begins by a single slender median chord from the hind
notch at the cerebral lobes, and immediately forms, or is in connexion with, a third slender
ring girting the oesophagus (ib. figs. 7 & 8), from the middle and upper part of which the
trunk-nerve of this system passes a short way back on the dorsal aspect of the stomach
before it divides. The divisions diverge at an angle of 45°, then bend, and are continued
* Philosophical Transactions, 1843, plate xii.
f Julus terrestris, outlined in my ‘ Anatomy of Invertebrates,’ p. 356, fig. 144, 8vo, 1855.
21
backward parallel with each other (fig. 8), running upon the dorso-lateral parts of the
wide and straight alimentary canal.
§ 5. Digestive System. — This system of organs includes a ‘ mouth,’ with instruments for
seizing and comminuting the food, a ‘ gullet,’ a e stomach,’ an ‘intestine,’ with vent and
accessory glands, of which, in the present genus, the ‘ liver ’ only has been recognized.
The mouth is median, and situated, as in other masticatory Crustacea, on the under
surface of the body ; it is, as in them, surrounded by modified portions of articulate limbs,
working laterally ; but these, in number and concentration, are parallelled only by the
extinct Merostomes (Cut, fig. 14, e. g .) : the mouth, it may be remembered, also resembles
that in Spiders in respect of its distance behind the fore border of the cephaletron (PI. II.
fig. 2). The circumoral integument, in Limulus, is yielding and elastic, cushioned out
with soft tissues, including fibres interlacing and susceptible, if muscular, of giving
change of form and position to the thick and prominent lips, endowing them with
movements, small in extent, but various, for seizing the morsels of food torn by the
haunch -palps or ‘carders’ (PI. II a. p, p). The thick labial epithelium yields to
such movements by transverse folds or indents. The mouth opens on a plane not only
behind that of the basal attachment of the antennules (or * first pair of chelate appen-
dages,’ n), but also clearly behind that of the basal attachments of the ‘ second pair,’ or
antennae (hi). Nor can those of the ‘third’ pair be said to be placed ‘posterior to
the mouth.’ Their nerves arise rather in advance than behind the oesophageal tube ;
and their haunches are on the transverse parallel of the anterior lip, as shown in
PI. II A. fig. 1, n iv, & fig. 2, p iv. In a general way the mouth of Limulus may
be said to occupy the interspaces of the haunches ( coxce ) of the right and left limbs,
iii-vii, these limbs being crowded or close-packed at their basal articulations, on each
side of the mouth, whence they diverge to their pincer-shaped tips. The haunches are
compressed, as if squeezed together; and their under or median borders are produced,
with a convex margin, which, with more or less of the contiguous flattened surface, is
beset with sharp, short, slightly curved spines. These are not mere processes of the
chitine, but are slightly movable, their base being articulated to a pit. The spiny plate,
or ‘ palp,’ of the first of these jaw-feet (in) is inclined backward, and overlaps part of
that of the second (iv), which has a like relation to the third (v) ; this is set more
transversely, and is wedged, as it were, between the second and fourth. The haunch
of this foot (vi) has a similar position between that of the third (v) and the somewhat
less spiny haunch of the last pair of legs or ‘ maxillipeds ’ (vn). This complex series
or circle of carding-instruments is bounded in front by the three-jointed antennae (n),
having the same chelate structure as in the multiarticulate ones of Pterygotus (Cut,
fig. 14) ; it is closed behind by the ‘ chilaria,’ or pair of appendages marked * in Pis. II a.
& IV.
The operation of these circumoral instruments in the living King-crab is thus described
by a close and accurate observer : — “ The food is held immediately under the mouth by
the nippers of the anterior pair of feet (n), aided, if necessary, by those of some of the
others. The manducatory limbs then begin an alternating motion of their haunches
upon the food, by drawing one of those rasp-like joints against the opposite one of the
22
same pair, the food being held between the two. This chewing by means of these
opposing rasps reminded me,” writes Dr. Lockwood, “ of the hand carding-process, in
which the card held by the right hand is brought towards and against the one held in
the left hand, the wool being between, when the right hand card is held still, and the
left hand duplicates the motion, and so on. The fine particles rasped off by the incurved
teeth pass into the mouth ” *.
The tumid and wrinkled margins of the mouth quickly contract to an oesophagus about
a line in diameter. This tube (PI. II a. fig. 2, ce) curves upward and forward in a
course of 1^ inch ; then dilates into a conical proventricular cavity (r) extending down-
ward, about 5 lines in depth by 3^ in breadth at the base. From the fore part of the
base a second short canal ascends, to terminate by a slight vascular prominence in the
stomach ( s ). The epithelium, or modified chitine, continued from the mouth along the
gullet and proventriculus, becomes suddenly thickened in the stomach, and is disposed in
numerous transverse ridges. The muscular coat of the stomach is concomitantly
strengthened, attaining at one part a thickness of 3 lines. The pyloric end (mt) projects
as a truncated cone, 4 or 5 lines long, into the dilated beginning of the intestine (i). The
truncate apex of the pyloric cone is slightly tumid. The epithelium lining that part has
resumed its thinness and subtransparency.
The intestinal tunics appear to be reflected from the base of the pyloric cone ; they
define a dilated beginning of the canal, and gain a slight thickness of the muscular coat
as they contract to the common size of the intestinal tube, the area of which is about
5 lines in transverse, and 3 lines in vertical diameter. The tube goes nearly straight to
the vent (PI. II a. fig. 1, i, v) ; but, about halfway there, it contracts transversely (PI. I.
fig. 1, i ), and exchanges its oval for a circular section, with a diameter of lines. Near
the vent it again expands, chiefly transversely ; and the muscular coat there gains some-
what in thickness. The vent (PL II a. fig. 1, v) is a transverse slit with tumid margins,
just anterior to the joint between the thoracetron and pleon.
The contents of the alimentary canal were pulpy and scanty. The principal food of
the Limulus polyphemus is stated by Dr. Lockwood {loc. cit.) to be Nereids, routed by
the cephaletral limbs out of the mud or sand displaced in the act of burrowing.
The only gland in communication with this canal is the liver. It is of great size ; its
minute terminal acini are compactly massed together, and occupy most of the space in
the cephaletron not given to other organs, mainly the generative, the ramifications of
which interlace with the hepatic lobes. A part of this mass is shown at n, fig. 1,
PI. I. ; but it extends forward to the space anterior to the stomach, and backward by
a narrow tract (ib. n') on each side of the intestine in the thoracetron. The lobes, or
larger groups of acini, form a close-packed series on each side, corresponding in the main
in number with the apodemal spaces and the epimeral nerves. The least unsuccessful
trials of injecting the terminal canals and acini indicated the greater transverse and less
longitudinal extent of the hepatic lobes or primary divisions of the gland (as shown
at n", fig. 1, PL I.). The gathering tubes of the initial or acinal ducts of these lobes
course, in the main, transversely toward the intestine until they quit the lobe, when they
* Lockwood (Rev. S., Ph.D.) in * The American Naturalist,’ vol. iv. p. 260 (1870).
23
converge abruptly to form the terminal duct. The anterior of these receives the tribu-
tary ducts of the four chief anterior divisions of the liver ; the posterior terminal duct is
formed by the union of the same number. The ducts of two or three of the anterior
lobes unite to form that which enters the main or terminal anterior duct ; those from
the four posterior lobes unite and enter the posterior terminal duct by two canals. The
arrangement, however, shown in the subject of fig. 1, PI. I., may be varied in other
specimens : but the principle of segmental constitution, as here exemplified by the
secondary ducts, will be found, I doubt not, in all Limuli. It indicates the liver to
have been developed, in relation to the primitive composition of the cephaletron
(Cut, fig. 5), of antero-posteriorly succeeding portions, there being a pair of livers or
hepatic lobes to each of five or more embryonal segments. The confluence of the ducts
interestingly exemplifies the way of subsequent concentrative growth characteristic of
the mature and procreative individual.
The bile is conducted to the intestine by two terminal ducts on each side : the first
pair (fig. 1, PI. I., & fig. 2, PL II a., I ) open upon the sides of the beginning of the tube,
where it contracts to the ordinary calibre ; the second pair (ib. m ) open about 9 lines
beyond, and nearer the dorsal part of the intestine.
As in the King-crabs, certain Spiders (Epe'ira, e. g.) have their ventral mouth f pro-
vided anteriorly with a chitinous plate, £ labrum ’ or £ prostome,’ and posteriorly with a
labium or * metastome,’ which is soldered to the cephaletral plastron, not bifid and movable
as in Limulus (Pis. II., II a., III., & IV.*). The oesophagus rises, at first, vertically
dorsad, then bends back at a right angle, traversing in that part of its course the neural
ring before expanding into the stomach. This cavity is, in most spiders, produced into
csecal appendages, which, in some, extend into the basal joints of the cephaletral limbs.
The bile-ducts open into that part of the intestine which traverses the thoracetron
(‘ abdomen ’ of arachnologists). The proportion of difference to resemblance must be kept
in mind when speculating on the degree of affinity of Xiphosura and Arachnida.
§ 6. Sanguiferous System. — The dissection of Limulus was commenced from that
aspect or plane of the body next to which, in Invertebrates, is the part of the neural
axis called £ superoesophageal,’ and which, as it supplies nerves to the organs of sense,
answers to the brain in Pishes. As in these Vertebrates, also the removal of the neural
or dorsal part of the skeleton (PL I. fig. 2) exposes the vascular system (ib. a, a) analogous
to the so-called £ aorta ’ of Pishes, and homologous with the £ dorsal vessel ’ in Insects.
In Limulus the walls of this vasiform heart exhibit muscular and valvular structures,
for the same purpose or office as those of the vertebrate £ heart.’
In a specimen dissected, with a carapace, or upper crust of the two chief parts of the
body, 9 inches in length, the heart was 4s inches 8 lines in length (Pl. I. fig. 2, a, a). It
was included in a delicate membranous sac analogous to a pericardium, but forming,
in fact, the wall of a venous sinus. This wall consists of two layers. One may be
properly termed a £ tunic it includes extremely delicate fibres, chiefly transverse, but
reticularly interwoven in a fine cellular bed, the inner surface of which has been the seat
of formifaction, or vital crystallization, of particles from the contained fluid, forming an
f a, fig. 109, ‘ Lectures on Invertebrata,’ 8yo, ]843.
24
epithelium *. The whole resembles a fine arachnoid membrane (a portion of this peri-
cardium is shown at b, fig. 2, PI. I.).
The heart is fusiform, widest at its hinder third, gradually narrowing, transversely, to
its fore end, which is 2^ inches from the fore part of the cephaletron, more rapidly con-
tracting to its hind end, which is 1^ inch from the joint of the tail-spine. In the vertical
diameter (PI. II a. fig. 1, r , r) the hinder contraction is more gradual.
The heart-wall consists of an outer, thin, smooth, compact coat, and a wall of striate
muscular fibres consisting of a thin outer longitudinal layer and a thicker transverse or
circular series. The wider part of the heart shows traces of an epithelial lining, due to
the action initiated or invited by a surface in contact with the formifying material in
solution. The arachnoid coat of the pericardial sinus is reflected over the outermost
proper tunic of the heart, and is continued into the venous ostia (PI. I. fig. 2, c , c), where
it gains thickness.
These ostia are sixteen in number, arranged in seven pairs at the sides, but towards
the dorsal surface, of the heart, with a terminal eighth pair. The hinder * ostia * are
rather nearer together than the others. The muscular tunic in the intervals of the ostia
(PI. II a. fig. 1, r , r) is about a line in thickness, but thins off rapidly at the two ends of
the heart. Each £ ostium ’ (ib/ o, o) is provided with a pair of narrow semilunar valves,
placed with the intermediate slit almost transversely to the axis of the cardiac tube.
The foremost artery (PI. I. fig. 2, h) runs to the £ ocelli ’ ( a 1), is there connected
with, or seems to enclose, the nerve ; it then bends down, following the curve of the
carapace to the angle formed by the upper with the flat under surface of the digging-
shield, near which angle the artery is reflected backward and cannot be further traced as
a distinct tube. On each side the origin of the £ ocellar ’ artery arises one of double the
size (ib. e, e), which, diverging from its fellow, curves outward and downward over the
fore part of the intestinal canal (PI. II a. fig. 1, s ) : it gives off, in this course, a branch
which ramifies upon the gizzard, a second to the intestine and liver, the main trunk
being continued to the nervous annular centre (ib. (3 ), where it expands, and combines
with its fellow of the opposite side to form a sheath for that centre analogous to a £ dura
mater.’ This rather loose sheath is continued along the ganglionic ventral chord, and
is prolonged, like a loose neurilemma, upon the nerves sent off therefrom, as it is upon
those in connexion with the annular centre.
Pine size-injection being thrown into the £ heart ’ from behind forward, appeared to
give a rich display of arterial ramifications. But dissection showed that the contents of
the seeming arteries ceased to be the simple injected matter where the £ gastric arches ’
(ib. s ) reached the nervous ring (ib. /3) ; for here the coats of the artery become thinned,
the injection lining them as a thin flake of colouring-matter, and, at the same time,
covering a thinner membrane which formed the £ neurilemma,’ or chief layer of the deli-
cate tunics of the nervous matter of the neural ringf. The same condition was pre-
sented by the seeming abdominal arterial trunk continued backward from the neur-
arterial circle (PI. V. a, b, to or. of nerve n 19). On slitting open the coats of the blood-
* Owen, * Anat. of Vertebrates,’ vol. iii. p. 499.
t Preparation, No. 1303 c, Physiol. Series, Mus. Coll, of Surgeons.
25
vessel, and washing out the flake of injection, the ganglionic nervous chord was exposed
in its interior *. The same result followed the like perquisition of the smaller ramifica-
tions of the vascular system into which the injection had penetrated, and engendered the
conviction that the main pair of arteries had hut a brief course as such +, becoming
resolved, on reaching the neural ring, into blood-sinuses — a condition which prevails
throughout a great proportion of the vascular system of Limulus. The whole nervous
system, save where the terminal twigs are lost in the tissues, is bathed in the blood of
these sinuses, which retain the appearance of ramified vessels, through their relations
to the nerves as the vascular envelopes of these $. Elsewhere the sinuses expand, lose
the character of tubes, or vessels, occupy the interspaces of viscera and muscles, initiate
from a subcardiac sinus the ramified branchial system of vessels, and return the blood
from all parts to the pericardial-like sinus enclosing the heart.
A pair of arteries is sent off near the anterior pair of ostia, and are closely connected
with the much larger veins emptying the neighbouring sinus into the corresponding
parts of the pericardial one. These arteries (PL I. fig. 2 ,f) pass outward and forward,
and subdivide into branches, which are lost upon the epimeral nerves. The next pair of
arteries correspond with the second ostial vein (ib. /') : I was unable to trace them far.
This vein courses outward near the hind border of the cephaletron, bends forward at n,
and runs parallel with the lateral or ocular ridge as far as the compound eye : its branches
are short, and speedily expand into sinuses §. A pair of arteries are obscurely indicated,
* The neurine thus seems to be small in proportion to the thick neurilemma, as Gegenbaur remarks ; but he did
not recognize the share taken by the arterial tissues in this sheath : — “ Beziiglich des feineren Baues soil die schon
oben angefiihrte dicke Umhiillung des Schlundringes erwahnt werden, derzufolge der eigentliche Nerventheil des
Schlundringes relativ klein erscheint.” Op. cit. p. 241.
t They are shown as cut off from the arches and lost upon the brain in Pl. V.
J This interesting stage in the differentiation of nerves and vessels was demonstrated in my Hunterian Lectures of
1852, ‘ Organization of the Entomostraca illustrated in the Limulus,’ Lecture xvi. Crustacea, ‘ Synopsis,’ March,
1852, and is briefly enunciated in the volume on Invertebrata as follows : — “ The sides of the great oesophageal
ring are united by transverse commissural bands : but the most remarkable feature of the nervous axis of this
Crustacean is its envelopment by an arterial trunk. A pair of aortas from the fore part of the heart arch over each
side of the stomach, and seem to terminate by intimately blending with the sides of the oesophageal nervous ring.
They, in fact, expand upon and seem to form its neurilemma ; a fine injection thrown into them coats the whole
central mass of the nervous system with its red colour.” — Lectures on the Comparative Anatomy and Physiology of the
Invertebrate Animals, by Prof. Owen, E.li.S. (second edition, London, 1855, Lecture xvi. p. 310). A similar con-
dition, requiring injection for distinguishing the vessel from the nerve, is pointed out in the Scorpion (op. cit. p. 449).
Gegenbaur, in his histological treatise on Limulus {op. cit. p. 241), remarks: — “ Auch die peripherischen Nerven
sind sammtlich von einer dicken Hiille umgeben, die sogar noch makroskopisch erkennbar ist.”
§ That an arterial canal accompanies the vein is indicated by the course of the blood, as observed by Packard in a
living larva of Limulus : — “ I could not see the walls of any of the arteries ; and indeed the arterial blood seemed to
flow in channels exactly like the venous sinuses, as in the arteries which pass around the margin of the carapace the
blood-disks were seen to pass by irregular currents towards the front edge of the margin. The anterior aorta
could not be detected in the young Limulus ; but on each side of the end of the heart the blood could be seen
rushing out and in, and with a general course downwards, beneath the oesophagus, while a current of blood flowed
on each side of the stomach and oesophagus, and thenoe went out at a considerable angle to the edge of the carapace,
where it divided, sending a branch around under the ocelli, and another along the outer edge of the cephalic shield,
and again subdivided opposite the second pair of cardiac valves ” (PI. I. fig. 2, c, f), “ one current following the
edge of the cephalothorax ” (ib. i n), “ and the other going on towards the heart ” (ib. b'). “ The abdominal arteries,
E
26
arising near the last pair of * * ostia,’ passing obliquely outward and backward. The
posterior or ‘ pleonic * artery (Pl. II. fig. 1, t ) has more definite tunics and bolds a
longer course than those from the fore part and sides of the heart. It is wavy at its
beginning, in relation to the varying directions of the tail-spine in its flexile movements
upon the body. The artery having entered the body of the spine, continues its course,
as such, along the dorsal side of the cavity, through two thirds of its length, hten sub-
divides and blends with the sinuses continued from the ventral chord and investing the
‘ cauda equina 5 of the tail- spine.
The veins, or venous sinuses with the least indefinite form, are those that course along
beneath the medi-lateral ridges of the cephaletron in association with the arteries
(PI. I- % 2, n, n ), and those which follow and lie near the margins of both cephal-
and thorac-etra. The latter return their blood by the posterior veins (ib. r ), united by
the median channel ( s ) with the pair in advance, q ; their common trunk opening into the
hind part of the pericardial sinus, b'.
§ 7. Respiratory System. — The gills consist of thin membranous plates of a broad semi-
oval shape ; there are from 150 to 200 in each gill or group, the number diminishing in
the hinder ones. The gills are in pairs, attached to the upper, hinder, or inner surface
of the proximal joints or broad coalesced plates of the last five thoracetral limbs (ix-xiii.
Pis. II., II. A, III. and IV.).
The branchial plates overlap each other from before backward. The anterior and
exterior one is the smallest; the others progressively increase to a little beyond the
middle of the series ; the hindmost again diminish, but in a less degree ; the whole
mass has the full oblong or irregular oval form shown in fig. 2, PI. V. Each plate is
strengthened by a chitinous filament along its free border, thickest where this is exposed,
so that the length of the gill is greater at its free or floating side than along its attached
base : the free margin is also ciliate.
Each gill-plate consists of two layers or membranes, united along the chitinous border,
. and also by numerous filaments so far apart as to divide the interspace into reticular
canals or cells, smallest at a subcentral space (fig. 3, a), and affecting a concentric
arrangement as they approach the free borders of the gill-plate. The two constituent
layers of the branchial plate may be regarded as productions or duplicatures of the
delicate skin of the upper or inner surface of the lamelliform limb.
Erom a venous sinus along the base of attachment of the gill-plates * the blood passes
represented by powerful currents of blood issuing from between the last two pairs of cardiac valves, are directed
obliquely outwards and backwards. The caudal aorta sends a current nearly to the tip of the spine, the venous
sinuses returning it along the sides. The simple arrows mark the course of the returning currents, which flow from
all parts of the body towards the valves.” — Development of Limulus polyphemus, pl. v. fig. 27, p. 171.
This admirable memoir appeared subsequently to the reading of my paper ‘ On Limulus ’ before the Linnean Society ;
and the Report given in the Number of ‘ Nature ’ for January 25, 1872, is quoted by Dr. Packard at p. 201. Dr.
Packard notes that the heart “ beats ninety times a minute,” in the larva after the first moult.
* “ II parait exister une libre communication entre ces diverses poches respiratoires ; car, en introduisant de Pair
dans une de ces duplicatures, on voit non seulement s’e'carter lcs lames de la meme branchie, mais meme se gonfler
toutes les branchies, ainsi que l’espace membraneux entre les pattes abdominales.” — V. der Hoeven, op. cit. p. 19.
The intercommunicating passage is the basal sinus, related to the gills, physiologically, as a ‘ branchial artery.’
27
freely into the interlamellar spaces ; whence it enters the vessels coursing along the
border of each plate, from the inner side towards the outer side (fig. 2, a) where the
vessel is largest. Here appears to begin the returning system of branchial veins on the
fore part of the base of attachment. These veins ascend and converge on each side of
the intestine, and traverse the pericardial sinus to enter directly the heart by the five
pairs of ostia at the widest posterior part of that organ.
The muscles which divaricate the hranchigerous limbs, and at the same time separate
the gill-plates and expand their cavities to the extent permitted by the interposed
columns, act as inspiratory ones, inviting the flow of blood from the abdominal sinuses
into the cavities of the gill-plates. This action may be supposed to take place when the
King-crab is moving or resting in its atmosphere of sea-water. The muscles which
approximate the hranchigerous plates and press them against each other and the thorac-
etron, will close the ciliate slits leading to the gills, will compress those organs, and
tend to squeeze the blood from the reticulate interspace of their constituent lamellae.
Such movement must be ‘ expiratory,’ and also effective in defending the delicate
surfaces of the branchial membranes from the atmosphere of muddy or sandy sea-water
when the King-crab is burrowing either for food or concealment.
§ 8. Reproduction of Parts. — Of the power of reproduction of limbs or other append-
ages, after mutilation, I have not found recorded evidence ; but such may be inferred to
be possessed by Limulus from the reproduction of the entire crust, as in other and higher
members of the class. According to Hr. Lockwood *, the King-crab moults several times
during the first year, perhaps five or six times between its exclusion in June to the
setting-in of cold weather. Like other Crustacea, it moults more frequently during the
earlier and more rapid period of growth than afterwards. The young Limuli acquire an
inch in length in the first year : it is then that, after the moult, the second pair of limbs
are observed to have gained the sexual form in the male. “ In older Limuli, just before
the time of exuviating, a separation occurs between the marginal rim and the perimeter
of the anterior shield.” “ To the unaided eye the rent is imperceptible, but opens on
exertions of the animal ; and at this opening it emerges from the old shell ”f. A specimen
which had accomplished this moult measured 9| inches in the short diameter of the
cephaletron, while the vacated shell was but 8 inches by the same measurement. This
exuviation was observed in the month of August : but Hr. Lockwood gives an instance
of a soft Limulus having been dredged up in the month of February, whence he thinks
they may moult twice a year.
§ 9. Generative System. — In Limulus the sexes are distinct ; the male is smaller than
the female ; and in both, the generative organs lack those accessory parts that relate to
intromission in some higher Crustaceans.
The ovarium is a system of ramified tubes and cavities, occupying chiefly the dorsal
region of the body ; it extends along the median part of the thoracetron, and expands
laterally in the cephaletron.
Parts of the ovary are single and median; the rest consists of parial symmetrical
lateral ramified tubes, chiefly situated in the cephaletron.
* ‘ The American Naturalist,’ vol. iv. No. 5. July 1870, p. 242. + Ibid.
E 2
28
The hindermost cavity (PI. IV. fig. 6, q) is a longitudinal tube, commencing by a
blind end above the rectum ; it extends forward, expands, and bifurcates about the
middle of the thoracetron ; the branches at first diverge, then bend inward and reunite,
sending back into the interspace of the bifurcation a short blind sac. From the base of
this heart-shaped portion the bifurcate tubes are continued forward, slightly diverging,
leaving a mid space for the heart and intestine as they cross the articulation between
the thoracetron and the cephaletron. About two inches in advance of the second
bifurcation each tube expands laterally into a triangular cavity, from the outer and fore
angles of which the ramified systems of the lateral loops, q", are continued. A small
branch is sent off from the outer side of the dilatation. Three or four tubes converge
from its fore part, and anastomose t to form the anterior single symmetrical cavity, q*.
This is oblong, subquadrate, subdepressed, and subreticulate. It is longitudinally chan-
nelled above, by the fore part of the heart resting thereupon, this part of the ovary
being interposed between the heart and intestine (PL IV. figs. 1 and 2, q). It seems to
have been developed in or from the last remnant of the included germ-mass. From the
hinder and outer angles of the antero-median part of the ovary proceeds the tube, which
passes outward and backward, joins that from the fore part of the lateral expansion, and
curves outward and forward to meet and inosculate with a similar retrograde branch from
the fore and outer angle of the antero-median lobe. From the outer side of these ovarian
loops (PI. I. fig. 2, q **, and PL IY. fig. 6, q") proceed four or five branches which inter-
ramify with the hepatic lobes. The branch tubes ( q *) continue from the fore part of the
antero-median sac ; and its loops are continued, subdividing and reticularly anastomos-
ing, along the sides of the gizzard to the fore part of the cephaletron.
Each of the main parial oviducal canals, before converging to the anterior reunion,
dilates and sends outward and backward a wide tube, which after sending off, or rather
receiving, three large tubes (t/**) is continued backward as the common oviduct (Pl. I.
fig. 2, o ; Pl. IV.fig. 6, o). The hindmost of the three large tubes passes outward and
backward to near the outer ends of the joint between the cephaletron and thoracetron,
and there curves forward beneath the lateral cephaletral ridge, and receives the ova from
the parts of the ovary extending to the lateral margins of the cephaletral cavity. The
foremost of the three branches collects the ova from the deeper-seated interapodemal
parts of the ovarium, the intermediate branch those from the dorsal level above and ex-
terior to the apodemata.
The numerical correspondence of the lateral tributaries to the main median or sub-
median receptacles of the ova with the neural indications of the segmental constitution
of the two chief divisions of the body, is less obvious than in those of the hepatic masses.
This may be due to the later period of development of the genital factories.
The part of the ramified ovarian system to which the term oviduct is here applied is
the tube, o, continued from the common stem of the three last-described tubes, and
f ‘ Lectures on Invertebrata,’ ed. 1855, p. 329 : shown, in Maia, in fig. 135, a', b'. Anastomoses between the
right and left system of ovarian-tubes were also noticed by Gegenbaur (toe. cit. p. 247), who well remarks on this
evidence of crustaceous affinity:—” Durch diese Verbindung beider Ovarialhiilften reiht sich Limulus an viele andere
Krustenthiere an, wo gleichfals ein unpaarer Abschnitt der inneren Genitalorgane vorhanden ist.”
29
passing backward, inward^ and downward, across the cephalo-thoracetral joint, to the
part of the upper or inner surface of the * opercular 5 limb, viit, shown in fig. 6, PL
IV. The termination here of the oviduct (p) was rather prominent : the outlet is trans-
verse, and formed by tumid labia, with the inner surface transversely plicate.
The bifurcation of the hind part of the ovary before passing from the thoracetron to
the cephaletron, relates mechanically to the accommodation of the cardiac and intestinal
tubes during the frequent and forcible inflections of the two great body-chambers upon
each other. The laden ovarium, instead of being pressed down upon the heart (as it
would have been if it had been continued as a single median and vertically parallel viscus
across the joint where the cephaletron was depressed at an angle with the thoracetron),
slips, by its division, on each side the heart during the inflection. A similar relation
to convenience of package governs the forward extension of the ovarian bipartition in
relation to the main parts of the heart and intestine.
The most significant difference between the female organs of Limulus and those of the
higher or malacostraceous squat-eyed Crustaceans is the absence of the dilated part of
the oviduct forming the copulatory pouch, or ‘ spermatheca,’ which absence relates to
there being no intromission in the act of impregnation in Limulus.
In the male, the testes are ramified and subreticulate, like the ovaria, and occupy
almost an equal extent of the two great cavities of the body. The sperm-ducts open upon
corresponding position of the opercular plate (PI. IV. fig. 8, p), their termination being
on a smaller but rather more prominent cone of thin yellow chitine, at the apex of which
the sperm-tube terminates by a whitish bilabiate orifice (PI. IV. fig. 7, b) *.
§ 10. Development. — It may not be unacceptable here to give the results of the
observations of the Rev. Sam. Lockwood, Ph.D., on the generation of the American
King-crab ( Limulus polyphemus ), condensed from the account he has consigned in the
under-cited periodical f.
In Rariton Bay, New Jersey, U. S., the King-crabs spawn in the month of May, June,
and July, at the periods of highest tides. In that operation they ascend from the depths
in pairs, the male holding on to the carapace of the female by his hook-feet (Pis. II.
& III. in). Arrived near the line of breakage of the highest tidal waves, “the female
digs a hole in the sand, and drops her spawn into it, upon which the male emits the
fecundating fluid, and the nest is then deserted, the parents returning seawards with
the retreating tide ” $. Occasionally a pair are left exposed by the tide, which they then
* Dr. Packard describes tbe spermatozoa as having a broad oval body, sometimes contracted before the anterior
end, and posteriorly suddenly terminating in a filament about four times as long as the body (‘ On the Development
of the Limulus polyphemus,’ 4to, Memoirs of the Boston Soc. of Nat. History, vol. ii. p. 156).
f * The American Naturalist,’ vol. iv. No. 5, for July, 1870.
+ lb. p. 264. Notes on the living Limulus are appended to the paper “ On the Relationship of the Xiphosura,”
&c., hy Henry "Woodward, Esq., E.G.S., communicated to the Geological Society December 20th, 1871 ; and in
reference to a remark by the author, that Crustacea, “ as a rule, appear to fecundate the ova by a true union before
the eggs are discharged from the ovaries,” the Editor of the ‘ Quarterly Journal’ (February 1872) refers to a paper by
M. Chantran, showing “ that the eggs of the common Crayfish are fecundated after expulsion from the oviducts ”
(p. 48). I gather, however, from that paper, that although ova may be impregnated after passing from the oviducts,
there is a more definite copulatory act than in Limulus, in which spermatozoa might find their way into the oviducts.
30
hasten to overtake if unmolested. By the action of the water the eggs, about half a pint
in quantity, are covered up with sand.
This thoughtful observer calls attention to the advantage of the choice of ‘ spring-tides,’
in the lengthened exposure of the sand-covered spawn to the vivifying warmth of the
sunshine during the ‘ neap-tides.’
On the 26th May, 1869, he obtained new-laid impregnated eggs, the hatching of which
was accomplished slowly, owing to “ the absence of those conditions of agitation, varia-
tion of water-depth, and sometimes complete exposure to air and sunlight, consequent on
the tidal flow” * *. July 18th the opaque exocliorion dehisced or cracked, disclosing the
white, pellucid, spherical endochorion. The included embryo consisted of two parts
(cephaletron and thoracetron) ; hut, as first observed by the masterly crustaceologist
Milne-Edwards f, the ‘ pleon,’ or tail-spine, was undeveloped.
Before hatching, the cephaletron is divided by an anterior mid dent into two lobes,
and the ocelli are not distinguishable ; its segmental constitution, or nature, is indicated
by six transverse linear indentations across the middle third ; the compound eyes appear
as pigment-specks outside the second and thud indentations. The smaller thoracetron is
triangular, with linear indications of the segments along the middle third of the dorsal
surface. The resemblance of the embryo Limulus, at this stage, to Sao is noted by
Dr. Lockwood ; but the cephaletral limbs are conspicuous, “ it has the feet quite ad-
vanced.” “ In the course of two or three days their extremities reach beyond the edge
of the carapace.” “ The embryo had its two segments inflected; and with short intervals
of rest (not many minutes at a time), kept up a very active revolving motion within its
pellucid prison ”J. August 3rd, seventy days from spawning, an embryo left the ovum.
It measured 2^ lines in length and 2 lines in width. Except for a little space in front,
“ the notch there being now obliterated and filled up by the part supporting the ocelli ”
(ib. a), the margin of the cephaletron is armed with spines, about twenty-five on each
side. The thoracetron is now nearly as broad as the base of the cephaletron, to which it
is articulated : its free border is semicircular, and provided with tufts of setae. The
growth of the tail-spike had not commenced. The liberated embryo “ at once began to
shift for itself, making a persistent effort to burrow like the parent.”
Such a spectacle was almost equivalent to a long retrospect in time — a watching of
the living Prestwichia, e. g. (Cut, fig. 17), on the old ocean-shore of Coal-brook Dale.
“The segmentary lines afford a very distinct trilobed character to both shields.” The
spiny and setaceous fringe finds its counterpart in Hemiaspis (Cut, fig. 18). “ In the
presence of the ocelli and the high-up position of the large sessile eyes, we have Eu-
rypterus shadowed forth ” § (Cut, fig. 13). “ The want of an articulated tail was soon
apparent in the case of our little Limulus. The slightest obstacle turns it on its back,
M. Chantran states that, in this act, “ the female lies on her back, bending forward the tail, and making a hollow,
into which the ova are passed, the male depositing the spermatophora upon the plates of the tail-fan and on the
plastron of the female, whose abdominal appendages secrete a greyish viscous fluid.” — ‘ Compte Eendu de l’Acad. des
Sciences,’ 15 Janvier 1872.
* ‘ The American Naturalist,’ vol. iv. p. 265. + ‘ Journal de la Societe Philomathique,’ Novembre 1S38.
{ Lockwood, he. cit. p. 266. § Id. ib. p. 267.
31
when, not haying this organ (which the adult uses so effectively in such emergency), the
little thing begins a vigorous flapping of the branchial plates. This causes it to rise in
the water ; then, by ceasing the agitation, it at once descends with a chance of alighting
right side up ” *. Eighty- two days after spawning, a young Limulus moulted. “ A few
minutes sufficed for it to withdraw itself from its hahy-suit ; in this act it rested a little
while, with the caudal appendage, now formed, only half withdrawn from the old
shell” f.
The extricated animal is J of an inch in width, and its tail is of an inch in length.
The tail is formed bent under the thoracetron, is at first curved, and “ requires some
hours to straighten out ”$. The setaceous fringe of the thoracetron is replaced by teat-
like or half-developed spines. The spiny fringe of the cephaletron is gone. The tail is
at first somewhat stumpy, almost ovoidal in transverse section, “ more distinctly marked
with lines of segmentation than is that of the adult ”§. As the young Limulus “tra-
velled on the mud before this moult, it made tiny rows of toe-tracks, leaving a plain
unmarked space between the rows. Now it moves with tail depressed, and makes a
median line, dividing the toe-tracks into two series ” ||.
The year following the collecting of the ova, Dr. Lockwood records the interesting fact
that certain ova at the bottom of one of the jars, “ which had never been in contact with
the sunlight,” still retained the embryo alive and revolving; these having been trans-
ferred to “ new sea- water and clean sand, with a good exposure,” were hatched ; and the
larval Limulus left the egg within two weeks of a year after oviposition and impregnation
of such egg %.
Dr. A. S. Packard**, from observations on impregnated ova of Limulus, transmitted to
him by Dr. Lockwood, adds details of intraovular steps of development, and gives accept-
able figures of these and of the excluded larva.
Pormifaction aggregates the protoplasmal beginning of the ovum into a central mass
P%. 1. Pig. 4.
Embryo with limb-buds
(Pkd. pi. iv, fig. 1).
of larger and denser granules, constituting the nucleus (cut, fig. 1, b), within which is
* Lockwood, he. cit. p. 263. + Id. ib. * Id. ib. § Id. he. cit. p. 269.
|| In quoting this observation, I am duly impressed by the “ caution for the interpreters of the ‘ Protichnites,’
seeing that the same species, at different ages, may make widely different tracks.” — Ib. p. 273.
if Id. ib. p. 272. This result recalls the arrest of development of Tadpoles kept in the dark.
** “ The development of Limulus poly phemus,” in Memoirs of the Boston Society of Natural History, vol. ii. 1872.
(This excellent memoir was read November 16th, 1870.)
32
the hyaline nucleolus, a. Round this is the mass of germ-yolk, c, in -which are recog-
nizable granules round other centres definable as yolk-cells, d. Impregnation of such
ovum is followed by denser blastodermal aggregates (cut, fig. 2, c). Peripheral differen-
tiation and condensation next define upon the blastoderm ( e , cut, fig. 3) a protoderm
(ib. p) within the chorion (ib. he).
The formation of the blastoderm accords with that of freshwater Gammari and of
Arachnids, the yolk not undergoing segmentation. It is not uniformly diffused, but the
seat of development is localized in an aggregate of more numerous and smaller blasto-
dermal cells (ib. e ).
The embryo is first recognizable, as such, by parial groups of these cells (cut, fig. 4)
on the surface of the protoderm (ib. pm; “amnion,” Brandt). These groups, by their
gradational difference of size, indicate, what later development shows, that they are the
beginnings of the cephaletral limbs. Between and a little in advance of the smaller
pair a round speck appears, which denotes the mouth. Portions of yolk (ib. x, x) are
detached before the formation of the blastodermic skin. This advance takes from one to
two weeks after exclusion under ordinary favourable influences, and is accompanied
by secession of the protoderm.
Fig. 5.
Embryo, with cephaletron and thorae-
etron defined. (Pkd. pi. iv. fig. 19.)
Fig. 6.
Embryo at the same stage, from below.
(Pkd. pi. iv. fig. 19 a.)
Fig. 8.
Newly hatched young.
(Pkd. pi. v. fig. 25 a.)
Embryo just before hatching.
(Pkd. pi. v. fig. 24.)
A similar heaping up of cells, as a ridge, marks out the hind margin of the cephal-
33
etron. The indications of segmental structure on its dorsal surface, or carapace, are mar-
ginal and transversely linear, due chiefly to the hepatic lobes seen through the transparent
skin ; they denote six segments (cut, fig. 5). The buds of the anterior pairs of thoracetral
limbs (ib. vm) next appear; and the joints of the longer cephaletral ones (ii-vnj
become more marked. The embryo now rotates in its moulted proto derm. The
definition of the thoracetron is speedily followed by the out-budding of a third pair of
limbs. The compound eyes (cut, fig. 7, i) appear as white dots; the ocelli (ih. a) are
next discernible on the first segment. Behind the six cephaletral segments there are
now eight thoracetral ones, and a ninth, pleonic, as broad as long. These are defined
upon the periphery of the spherical embryonal mass.
At this stage the chorion cracks ; and sea- water, endosmotically filtering through the
protoderm, expands it, and allows free flotation to the rotating embryo. The heart
appears as a pale streak, extending from the front edge of the cephaletron to near the
opposite end of the thoracetron, along the median dorsal depression. The reaction of
the sea- water upon the intra-ovular embryo, combined with excentric pressure through
growth, is manifested by the peeling off of a thin skin. The body becomes flattened as
it broadens ; the median region of the tergurn rises, and interrupts the segmental lines ;
the compound eyes project from the boundary ridges between the median and lateral
regions, and the three-lobed character of the carapace is manifested. The ‘sternal’
surface recedes from view, in profile, and the hollow (cut, fig. 8, a) lodging the mouth
and maxillipeds begins to be established. At this stage the spatulate appendages of
the penultimate joint of the limb vii. appear as simple spines, and the terminal forceps
is complete in this, as in the antecedent limbs. Now, also, the £ chilaria ’ appear as
rather flat oval tubercles closing behind the sternal or ‘ oral ’ groove (fig. 8, *).
In this state of development the young Limulus escapes from the * protoderm ’ (amnion,
endochorion). The cephaletron is about half as long as wide, its margins are fringed
with cilia, from pits on their upperside. About three weeks after hatching, the skin is
shed ; the thoracetron shows its marginal notches and movable spines, the latter shorter
than in the adult. A fourth pair of lamellate limbs appears. The pleon now projects
from the mid notch of the eighth segment, its base embracing the vent, which opens
upon it ; its apex is subacute, and its length about thrice its basal breadth. A second
moult was observed between the middle and latter end of August.
The sum of these observations shows the progressive acquisition of the mature cha-
racters of the King-crab without undue development attended with subsequent loss or
curtailment of parts in relation to a phase with habits of life markedly different from
those of the adult — in other words, without * metamorphosis.’ In this respect Limulus
follows the course shown in Astacus fluviatilis f and some other Crustacea, as well as in
Arachnids and Cephalopods.
Dr. Anton Dohrn $ has also recorded notes on the ovum, embryo, and young of
+ Comp. fig. 4 with fig. 136, p. 336, ‘ Lectures on Invertebrata,’ and fig. 5 with fig. 137, p. 337, ib.
* “ Untersuchungen fiber Bau und Entwickelung der Arthropoden,” ‘Jenaische Zeitschrift,’ Band vi. Heft 1
(1871), p. 582. Of other contributions by this excellent observer to the embryology of the Crustacea I may cite : —
‘Die embryonale Entwicklung des Asellus aquaticus,’ 8vo, 1867 ; ‘ TJntersuchng. fib. Anat. u. Entwicklg. d. Arthro-
F
34
Limulus , afforded by specimens preserved in alcohol, transmitted to him by Dr. Packard.
They are confirmatory of the accuracy of the observations of the able American em-
bryologist as detailed and illustrated in the work above cited. Dr. Dohm premises
a German translation of Dr. Lockwood’s memoir in the ‘ American Naturalist,’ and of
the ‘ Abstract ’ (which appeared in the same periodical) of Dr. Packard’s Memoir. The
chief characteristic of the contribution by the German carcinologist lies in the point
of view which he has taken of the phenomena. It is a development of that sketched
out as follows, in my ‘ Lectures on Crustacea ’ of 1843 and 1858. “ To what end,
it may be asked, tends all this discussion concerning the affinities of animals that
have long ceased to exist ? How are we concerned with it in considerations relative to
the generation and development of the actual Crustacea ? To this I have to answer, that
it is only by a knowledge of the transitional larval forms of these that we come rightly
to comprehend the nature and affinities of the extinct Trilobites, and that our knowledge
of the most interesting relations of actual larvce requires a ’previous knoicledge of the
forms of their class that have heretofore existed on this planet ” * * * * §. This view is developed
and illustrated, with large assumptions, as follows, by Dr. Dohrn : — “ Pritz Muller him-
self made the first decided application of this ‘law,’ viz. that the embryological develop-
ment was nothing more or less than a short, though not always exact, recapitulation
of the history of all the ancestors of the organism in question — by tracing the different
orders of the Crustacea back to their common ancestor, the famous Nauplius, that
little crustacean larva that quits the egg and is afterwards gradually developed into the
well-known diversified and more highly organized forms ” f.
It may not be out of place here to recall what is understood in plain matter of fact by
the term Nauplius, as contrasted with its transcendental signification.
The young of Entomostraca, with ciliate natatory limbs (cuts, figs. 9 & 10) more or less
like those of the parent, want, when hatched, the protective bivalve-like cephaletral
shield and some other parts of the adult, yet soon show characters which enable the
student of the group to refer them to their species, the full diagnosis of which they yield,
as in Limulus , after successive ecdyses.
The first systematic observer of the small representatives of the subclass t, not knowing
the genetic relations of his subjects, referred the young of some species to distinct genera
— those of Cyclops ( Canthocamptus) minutus e. g. to a genus Amymone , and the hexapod
stage of Cyclops quadricornis to a genus Nauplius. Later observations have led to these
larvae being relegated to their proper genera and species §. Nauplius saltatorius, O. E. M.
(fig. 10), is the young of Cyclops quadricornis-, Nauplius hipes (fig. 9) is the larva of Apus
poden, I. Cumaceen,’ 8vo ; ‘ II. Pycnogoniden ; ’ ‘ III. Daphniae,’ 8vo, 1869 ; ‘ Die SclialendrUsc u. embryon.
Entwieklg. d. Daphnien,’ 8vo, 1869 ; ‘ Ueberreste d. Zoeastadiums in d. ontogenet. Entwickel. d. verscliied. Crustac.-
Fam.’ 8vo, 1870 ; ‘ Enters, iib. Bau u. Entwick. d. Arthropoden,’ I. & II., 8vo, 1870.
* ‘ Lectures on Iiivertebrata,’ 8vo, ed. 1855, p. 333.
t Dr. A. Dohrn, in the ‘ Academy ’ for Nov. 1, 1871, p. 429.
i ‘ Entomostraca, seu insecta testacea, quae in aquis Daniae et Norwcgiae reperit, descripsit et iconibus illustravit,
Otho Fred. Muller,’ 1785.
§ See the excellent work ‘The Natural History of the British Entomostraca,’ 8vo, 1850, by W. Baird,
M.D., F.L.S.
35
cancriformis : other Nauplii and Amyruonce are entered among the synonyms of the full-
grown parents to which they severally belong — as, e.g., to Canthocamptus minutus, Baird,
C. stromii, C.furcatus, C. chelifer, &c. The c Nauplius ’ of B alarms, i. e. the young of that
Barnacle after quitting the egg, is a free e hexapod,’ with relatively larger swimming-
limbs, furnished with more numerous and relatively longer setae than in N. saltatorius *.
Nauplius (young of Apus ) cancriformis.
(After Baird, op. cit. tab. i. fig. 2.)
Fig. 10.
Nauplius saltatorius (young of Cyclops).
(After Baird, op. cit. tab. xxiv. fig. 9.)
Thus it appears that Nauplius is not a e thing ’ hut a * name.’ That is, the term
means not one hut many things, and all of them known more truly or scientifically under
other c nomina ’ of multitude, both generic and specific.
It is essential in this part of my memoir to hold an intelligible idea of what is signified
by Nauplius, in reference to its application to the question whether the embryonal de-
velopment of Limulus is a “ recapitulation of the history of all its ancestors,” or merely
a manifestation of the phases of its own specific growth — and if the latter, whether any
of those phases resemble not a Nauplius only, hut other species or genera of Crustacea,
more, and in more essential characters, than they resemble later phases or the generic
characters of the parent.
At the phase of development of Limulus (fig. 4) which is called the ‘ Nauplius stage ’ f ,
the resemblance is as follows : the limbs are restricted to the part of an undivided
body answering to the later-defined cephaletral division, as yet not distinctly marked
out. The correspondence of the embryo Limulus to the young Entomostracan is
carried no further. The cephaletral limbs in the former are mere buds ; the terminal
joint is bent on the proximal one ; there is no trace of setae, not the slightest indication
of any transitional natatory structure or function of such embryonal limbs. The mouth
opens, almost, in its limuline relations to the antennules (n) and antennae (in) ; and
these already show their characteristic difference of size. Their next step is to gain the
prehensile chelate structure, as in the adult. What the “ famous Nauplius ” may he
I have not been able to make out ; but if the stage in question really represents any
“ common ancestor,” it certainly is not the Nauplius of carcinologists. It may also be
remembered that Limulus differs from the parents of Nauplii, i. e. Copepods, Phyllopods,
and other Nauplian Entomostraca, in the eggs being left to hatch in a sand nest, not
carried about in egg-hags.
* C. Spence Bate, “ On the Development of the Cirripedia,” in ‘ Annals and Magazine of Natural History,’ 2nd
Series, vol. viii. (1851) p. 324, pi. vi. fig. 1, Balanus balanoides ; fig. 5, Balanus perforatus.
f Packard, loc. tit. pp. 163, 202,
F 2
36
Shortly after the foregoing so-called “ Nauplius-stadium,” the thoracetral limbs begin
to show ; and this is termed the “ Zoeal stage.” The phenomena supporting or suggest-
ing that phrase are, that in the Limuline larva both cephaletron and thoracetron are de-
fined, with limbs, and that the pair of compound eyes are discernible on the former. But
the Zoea of the Brachyura * is framed, like the Nauplius of
the Entomostraca, for free natatory life. Its limbs are exclu-
sively * cephaletral,’ and are terminally branched and ciliate.
The thoracetral segments show no limbs ; and the terminal
or ‘ pleonal ’ one is bifurcate and ciliate, for assisting the
parial limbs in swimming (cut, fig. 11).
At the subsequent, so-called, “ Trilobite stage” (fig. 7) t,
the young Limulus has a superficial resemblance to some
of the Trilobites, and especially when these are at perhaps a
corresponding period of development. The body of the so-
called larva e. g. of Trinucleus ornatus (cut, fig. 12, a) consists
of two shield-like and somewhat semicircular-shaped parts
joined together by their truncate or transverse borders. The
upper surface of the foremost, answering to the ‘ cephaletron ’
of Limulus, has also a raised median region defined from the
two lateral regions — a configuration which suggested the term
‘Trilobite. ’
But here the resemblance ceases in the main.
The hind division in the Trilobite (fig. 12, c) is not the homologue of that of the larval
Limulus. The ‘ thoracetron ’ of Trilobites (ib. b) is developed, like the supernumerary
Fig. 12.
segments added to the primary ‘ eight ’ in Julus, by successive formation in the germinal
space between the cephaletral (ib. a) and pleonic or pygidial (ib. c) divisions of the body.
The cephaletron of the Trilobite has no articulate appendages. It is doubtful, to say the
least, whether any were attached to the thoracetron (ib. b). What have been supposed
to be such in that part of Asaphus platijcephalus are not lamelliform, operculate, or
* Anat. of Invertebrata, 1855, p. 340, figs. 138, 139.
f “ Das Stadium welches wir jetzt betrachten wollen, kbnnen wir am Besten und Bezeichnendsteji das ‘ Trilo-
bitenstadium ’ benennen.” — A. Dohrn, op. cit. p. 588.
+ By E. Billings, Esq., E.G.S., Quarterly Journal of the Geological Society of London, vol. xxvi. pi. xxvi. fig. 1
(May 1870) ; also by H. Woodward, Esq., F.G.S., Geological Magazine, vol. viii. (1871) pi. viii.
37
branchigerous at any period of the Trilobite’s existence ; but, if the ridged (what a car-
penter would call { beaded ’) inferior borders of the eight thoracetral segments have not
been so misinterpreted *, are slender, filamentary, cylindrical, jointed ambulatory limbs,
terminated by a claw +. Under either alternative the difference is great as compared
with the coalesced pairs of broad lamelliform articulate appendages of the thoracetron of
the larval (fig. 8) as of the mature Limulus (Pis. II. & III.), and still greater when the
Trilobitic larva, with its pleon or pygidium for the second hody-part, (fig. 12, a, b), is
compared with what is termed the “ Trilobitenstadium ” of the Limulus ; in which
stage one sees, with the thoracetron for the second body-segment, beneath it already
developed three or more of the lamelliform limbs, on the second and third of which the
gill-plates have begun to appear {. This is far from being a ‘Trilobite;’ and nothing
is gained to science by putting figurative expressions for facts. In the inductive school
of biology, the notion that a higher form traversed a series of lower forms in the course
of its development has ceased to be set forth, save under duly modified terms §. I am
under the impression (and it is an agreeable one to the mind searching solely for intel-
ligible and demonstrable conclusions) that few now dispute the fact that each individual
of a given species is such ab initio , and takes its own course to the full manifestation of
its specific characters, agreeably with the nature originally impressed upon the germ.
A King-crab does not, any more than a perch, a dog, or a man, begin to be such only
when the zoologist discerns the respective characters of the parent, but is such even
before embryologists detect their earliest dawn. The embryo Limulus derived its nature
and the potency of growth according to the specific pattern from the moment of the
impregnation ; and each step of development moves to the consummation of the pattern
as its end and aim || . The generic character is indeed significantly soon shown in the
budding Limulus.
The first steps, like those in all segmental (whether articulate or vertebrate) animals,
recall the work of crystallization, and illustrate growth by repeated samenesses. These
show the results of formifaction, aggregated in series of similar heaps of organic atoms
(fig. 4) before the specific affinities begin formally to operate thereon and plainly to
show themselves to the eye. No sooner, however, can one of these heaps, or pairs of
heaps, be recognized as budding limbs, than in such series the first is seen to be Limuline
by its halting growth (fig. 6, ii) ; the second (in) pushes on outside these, the basal
joints of the ‘ antennules ’ (ii) being at the interspace of those of the * antennae ’ (fig. 8,
m), according to the King-crab’s pattern.
[Further back in that interspace opens the mouth (fig. 6). It is at no developmental
stage typical as a transient manifestation of the ordinary position of the mouth in an
annulose animal; that is to say, it is at no time terminal — but as soon as it opens
(fig. 4), testifies by its inferior position that it is the mouth of a Limulus, not of any other
or any lower form.
*' I offer this alternative with diffidence, as I have not had the opportunity of examining the exceptional specimen.
f According to H. Woodward’s restoration, in ‘ Geological Magazine,’ July, 1871, pi. viii. fig. 1 a.
J Packard, loc. cit. p. 170, pi. v. fig. 26.
§ See the concluding Lecture of my course on “ Invertebrata,” of 1843, 8vo, p. 367.
|| Anat. of Vertebrates, vol. i. p. xxi.
38
Thus, in the existing representative of Xiphosura, the embryo or larva is neither a
Nuuplius nor a Zoea, nor a Trilobite : it is a Limulus, exhibiting the characters of such in
stages of development or growth corresponding to the period of incubation at which the
immature creature may be examined.
§ 11. Conclusion. — That the Trilobite, like the Limulus , possessed articulate limbs,
has, however, been advocated not only by interpretation of appearances
in an exceptional instance, but by appeal to the laws of coexistence *.
I would submit, however, the following remarks bearing upon the cor-
relation of vision with other ways and means of locomotion.
Limulus possesses the pair of relatively large compound eyes, set high
upon the lateral parts of the cephaletral carapace ; and besides these, it
has the pair of small anterior simple eyes : it looks forward and upward,
and commands, like the guns of a demilune bastion, a like range in the
horizontal sweep. And yet the prevalent impression, from the position
and proportions of its subcylindrical jointed limbs, is that they subserve
the needs of digestion much more than those of locomotion. No observer
has yet testified to their capacity of uplifting the body from the ground,
whether dry or submerged, and of bearing it along by successive steps,
as do the jointed legs of the Isopod, the Lobster, or the Crab. Some of
the pairs are obviously incapable of such locomotive functions. The last
pair (vn. in all the figures) may help to push the body along the sand,
as the oar serves to shove off a boat ; but that is all.
Eurypterus (fig. 13) and Fiery gotus (fig. 14) possess, like Limulus, both the antero-
median ocelli and the medio-lateral compound eyes. In Pterygotus the antennae are
forcipated members for prehension of food, as in Limulus . The
three succeeding pairs of cephaletral limbs are still less capacitated,
through their proportionally smaller size and concomitant slender-
ness, for gradatorial movement of the body. They are adapted to
rout out of the sand or mud, disturbed by the spade-shaped head,
the objects of food which the front pair is modified to seize. The
larger terminal pair of limbs are more decidedly natatory in form
than are the last cephaletral lamelligerous pair (PI. II a, figs. 2, 4, vn)
in Limulus. In Eurypterus the cephaletral limbs anterior to the
lamelliform natatory pair seem to be alike in structure, unless the
antennal forceps has been wanting in the fossils, and must have
had functions as limited as are their size and strength.
These considerations weigh with me in checking a tendency to
conclude that the Trilobites, because they had large compound eyes,
Pterygotus anylicus, Ag. must have had articulate ambulatory limbs of as strong a texture,
Eurypterus Scou-
teri, Hbt.
Fiar. 14.
* « The large compound sessile eyes, and the hard, shelly, many-segmented body, with its compound caudal and
head-shield, differ from any known Phyllopod, hut offer many points of analogy with the modern Isopods ; and one
would be led to presuppose the Trilobites possessed of organs of locomotion of a stronger texture than mere bran-
chial frills.” — H. Woodward, Geological Magazine, vol. viii. p. 523.
39
whether crustaceous or chitinous, as their body-segments. That sixteen slender freely
movable filamentary limbs, as restored by Mr. Woodward* * * §, each nearly 1J inch in
length, attached by a flexible joint no bigger than a pin’s head, and divided into
seven movable segments by six other joints, in a Crustacean that may have under-
gone, to say the least, some disturbance between death and fossilization — that the eight
pairs of such articular appendages should remain and be found symmetrically and
regularly arranged across the ventral surface of the fossil, with intervals, if not parallel to,
yet corresponding in length with those of the thoracetral segments — presents itself to my
mind as much less probable than that the narrow parallel ridges which constitute
the observable phenomena should have had such extent of attachment to the ventral
surface of the several segments as to offer the requisite physical resistance to displace-
ment and to loss of original regularity and symmetry of position, such as the specimen
of Asaphus platycephalus described and figured by Billing + actually presents to view.
If this Trilobite possessed the ambulatory legs ascribed to it, it could hardly be an excep-
tion, in this endowment, to its order, and traces of such limbs, in divers conditions of
displacement, would be common.
The varied and usually more or less dislocated positions of the jointed limbs in the
fossil Merostomatous crustaceans would lead one to expect a like condition in other
families of palaeozoic fossils possessing similar appendages J.
The difficulty of getting a clear view of the nature and affinities of Limulus at the stage
of anatomical investigation which had been reached before the date of the present paper,
and the need of such further help as could be given by one occupying himself therewith
by the way, as it were, and in the brief snatches of leisure which administrative duties
and the cultivation of more congenial fields of original research might permit, will be
appreciated from the fact that one who has devoted to this question so much pains, and
skill, and dialectic ability as the indefatigable crustaceologist Dr. Anton Dohrn has left
his conclusions as to the class-characters e. g. of Limulus in a condition, to say the
least, not so supported as to command the common consent of his fellow labourers.
Bor myself it is a plain duty, and under responsibility for opportunities of dissection so
kindly and liberally afforded by American friends, to give my reasons for dissenting from
the view of Limulus being so far Arachnidan as to require, with its extinct allies, to
be placed as a distinct group, not of, but by the side of, the Crustacea §.
* Geol. Mag. viii. 1871, pi. viii. fig. t a. + Loc. cit.
J The above considerations incline me to view, as the more probable interpretation of the appearances in this fossil
that given by the accomplished naturalist Dana, to whose writings, and especially those on the Crustaceous class,
I am indebted for much interesting and valuable knowledge.
§ “ Limulus ist zunachst verwandt mit den Gigantostraken ; heide erscheinen verwandt mit den Trilobiten, ohwohl
diese Verwandtschaft nicht in alle Details nachgewiesen werden kann. Die morphologisch-genealogischen Bezie-
hungen dieser drei Familien zu den Crustaceen lassen sich vor der Hand nicht feststellen, bleiben vielleicht fiir immer
zweifelhaft. — Sonach hleiht uns nur iibrig, diese drei Familien unter einem gemeinsamen Namen, wofiir ich Hackel’-
schen Ausdruck ‘ Gigantostraka ’ mochte in Yorschlag gebracht hahen, selbstandig zu constituiren und im System
nehen die Crustaceen zu stellen.” . . . “ Was bei Savigny andeutungsweise, hei Strauss-Diirckheim mit Einseitigkeit
ausgesprochen wurde, das tritt also jetzt unter dem Gesichtspunkte der Deseendenztheorie von Neuem auf. Die
Yerhindung der Arachniden mit den Crustaceen soli durch Limulus und die ihm verwandten Eurypteriden gegeben
40
In these questions the nervous system yields important indications. If it were a fact
that “ in Limulus only the foremost pair of limbs was innervated from the superceso-
phageal ganglion, the rest deriving their nerves from the abdominal ganglionic
chain the advocate for its elimination from the Crustaceous class would have an
argument of weight for the affinity of Limulus and its extinct allies with the Scorpion
and Spider.
The allies here referred to are those possessing cephaletral limbs the general characters
of which are repeated in Limulus.
The anatomical investigations of well-preserved mature King-crabs, the results of
which are given in a previous section (§ 4) of the present memoir, have convinced me
that Limulus, like other Crustacea, does derive the nerves of its two anterior pairs of
cephaletral limbs (n, in) from the cerebral (=superoesophageal, here prae oesophageal)
ganglion. The portion sending off the nerves of n. and ill. is not, indeed, so distinct
from the rest of the neural circle as in Astacus ; but it holds the same relative position
to the gullet. It is even within the bounds of fact to say that the origin of the nerves
of iv. is nearer the fore than the hind part of that canal. Save at the price of making an
arbitrary section, and imposing an illegal or unnatural boundary line, no one can contend
against the Crustaceous nature of Limulus on the score of alleged suboesophageal origin
of the antennal nerves, or those of the limbs (hi, in the Plates of the present Memoir).
If Dr. Anton Dohrn be not prepared to pay this price, the analogies or resemblances
indicated by Strauss-Diirckheim, Savigny, and Latreille, of Limulus to certain Aracli-
nidans, will not suffice to outweigh the type of generative organs and extraneous
impregnation, combined with the aquatic respiration and branchial organization of the
present Condylopod and its palaeozoic allies.
I fully concur with the estimable and experienced naturalist Tan Beneden, that
branchiae of themselves may be an artificial class-character. But I cannot suppose that the
incipient or larval relations of the nervous centres to the nerves are essentially different
from those unquestionably demonstrable in the full-grown Limulus. The idea, therefore, of
all the limbs succeeding the antennules (it) being supplied from the abdominal ganglionic
cord, must be laid to the acknowledged difficulty which Anton Dohrn met with in
tracing out their several relations in the embryo Limulus 2-3 lines in length f, trans-
mitted to him preserved “ in strong whiskey.” Admitting, then, Limulus to be a
sein.” — P. 638. [What Savigny has indicated and Strauss-Diirckheim has partially (one-sidedly) expressed, re-
appears now under the light of the theory of evolution (descent) — that the connexion of Arachnids -with Crustaceans is
given hy Limulus and the allied Euryptendce. Limulus is most nearly allied to the Gigantostraca ; both appear to be
allied to the Trilobites, although this affinity cannot be shown in all details. The morphologico-genealogical relations
of these three families to the Crustacea cannot be stated at present, and will remain, perhaps, always dubious. At
present we are entirely unable to say any thing of their relations to the Arachnida. Consequently only one course
remains for us, viz. to form an independent group for these three families, with a common name, adopting that of
Gigantostralca proposed by Haeckel, and to place it in the system at the side of the Crustacea.]
* “ Dei alien Krustern empfangen namlich die beiden vorderen Extremitatenpaare ihre Nerven aus dem oberen
Schlundganglion. Bei Limulus aber wird nur das vorderste Paar der Gliedmaassen von dem oberen Schlund-
ganglion versorgt, die iibrigen empfangen ihre Nerven aus der Bauchganglionkette.” — A. Dohrn, loc. cit. p. 585.
f Op. cit. p. 586.
41
Crustacean (incipient, it may be), what are its nearest allies in that class ? Do the
grounds on which I reject a ‘ Trilobiten-Stadium ’ at any period of its larval life meet
with any support from affinities manifested by the adult to other Crustaceous forms ?
Pterygotus and Lurypterus resemble Limulus in the organs of vision, save that the
facets of the large lateral compound eyes are less distinct or less conspicuous in the fossil,
possibly exuvial, specimens of those extinct forms.
Both palaeozoic genera manifest a clear and exclusive affinity to Limulus in the general
proportions, modifications, and functions of the cephaletral limbs. In Fig. 15.
Pterygotus (fig. 14) the foremost pair is chelate, the hindmost pair
lamellate, the intermediate pair are less differentiated and are alike.
In Slimonia (fig. 15) the foremost pair is the smallest and shortest,
the hindmost the longest, and it is also lamellate. In both genera all
the cephaletral limbs, at least all but the foremost, had the basal joints
beset with 4 carding-spines,’ showing their functional subserviency, as
in Limulus, to the mouth as preparatory organs of digestion.
We may consequently infer, from the analogy of the food of the
living King-crabs, that Nereids and other soft-bodied Annelids abounded
in the sandy or muddy beds of the old ocean in which the Merosto-
mata * burrowed.
In these the cephaletral shield (figs. 13, 14, a) was small, both in
♦ breadth and length, as compared with that in Limulus , Prestwichia
(fig. 17), and Bellinurus\ but it was similarly shaped as regards the
curved anterior trenchant fossorial margin. The mouth was inferior, nata, Wd.
bounded laterally by the carding-joints, and posteriorly by a 4 labium,’ or connate chi-
laria, of large size — and, if homologous with the parts in like relation to the mouth of
Limulus (*, *, Plates II., II. a, III., & IV.), differing therefrom by the pair continuing the
condition shown by the thoracetral limbs of Limulus. Whatever homology be adopted,
the hindmost of the 4 trophi,’ or oral organs, is single and symmetrical in Eurypterids.
No fossil Merostome has yet been discovered showing more than three pairs of cephal-
etral jaw-limbs between the foremost and hindmost pairs. Thus there is one pair less
than in Xiphosures (fig. 8). In the fine fossil exuvium of the young Pterygotus cmglicus
figured by H. Woodward in his excellent ‘Monograph on the Merostomata’ (plate ii.
fig. l)f, there seem to be as many as five limbs on the left side, with spinigerous
haunches ; but it is uncertain whether the foremost of these may not be the fellow of the
second, displaced from the right side. This, therefore, leaves the forcipate antennae, or
foremost pair of jointed cephaletral limbs, devoid, like that pair in Limulus, of the basal
carding jaw-plate.
I think it of less moment to speculate as to which of the six pairs (ii-vii) of cephal-
etral limbs in Xiphosures were undeveloped in Eurypterids, than to realize the certain
correspondence of character of the five developed pairs in the latter family with those
* This term signifies, and most aptly in its present adopted extent of application, the peculiar structure and func-
tion of the cephaletral limhs described in previous paragraphs.
f Yol. of the Palaeuntographical Society for 1866.
G
42
attached to the Limuline cephaletron. As in Limuhis , moreover, a partially coalesced
pan of opercular plates extended backward in Pterygotus from the under and hinder
border, more or less concealing the underparts of the two anterior segments of the
thoracetron (fig. 14, b).
The foremost division of the body (a), in both Slimonia (fig. 15) and Pterygotus, is
composed of fewer segments than in Limulus. The next division of the body, b, in Eury-
pterids, includes a greater number of segments ; and the broadest of these but little exceed
in that dimension the coalesced cephaletral segments. The pleonal ones, transitorily
manifested at the basal part of the telson in Limulus , retain their individuality in Eury-
pterids, so that the distinction between thoracetron, b, and pleon, c, is arbitrary, and
only the telsic termination of the third division of the body is definable. Accordingly,
the whole body of the extinct Merostomes is longer and narrower, exhibiting less of
concentration and more of irrelative repetition, than in the existing Limuline form.
If shape and relative size affect so little the conclusion above supported of the homo-
logy of the cephaletron in Xiphosures and Eurypterids, much
weight cannot be attached to the difference of form and propor-
tions of the £ pleon * or * telson 5 in the same question, especially
with present knowledge of the intermediate modifications of this
division of the body, as seen in Eurypterus (fig. 13)* and Stylon-
urus (fig. 16). As the cephaletron of Limulus includes more
segments and appendages than does that of Pterygotus, so like-
wise may the pleon of Pterygotus as compared with that of Li-
muhis. The excess of segments of the thoracetron in Eurypterids
(which excess H. Woodward is disposed to refer to another divi-
sion of the body, which he terms * abdomen ’) may be among
those of which embryologists of Limulus believe themselves to
have seen traces in its budding tail-spine. However that may
be, or be accepted, the pleon or telson in all Merostomes is ter-
minally pointed or spinous, and would help in the movements of
the animal much in the same way as Lloyd and Lockver have
observed it to act in Limulus.
That this tail-spine (pleon and telson) is a serial homologue, reduced and simplified,
of the segments, and not in the category of limbs or other mere appendages, the modifi-
cations thereof in some of the extinct allies and predecessors of Limulus give evidence of
weight. The argument for its appendicular grade, from time, “that it is developed sub-
sequently to the other segments,” can only apply on the assumption or supposition that
all true segments or £ somites ’ of a Crustacean are simultaneously developed. The state-
ment that the £ tail-spine 5 is developed not only subsequently to, but “ from the dorsal
surface + ” only of the body, has but the value of an unsupported assertion. If the attach-
* Nieszkowski, “ Der Eurypterus remipes aus den obersilurischen Schichten der Insel (Esel.” Archiv. fur die
Naturkunde Liv- Ehst- u. Kurlands, Erste Serie, Bd. ii. tab. i. fig. 1"; quoted by Anton Dohm, Joe. cit. p. 640, Taf.
xiv. fig. 21. H. Woodward, Euryptems brodiei, from Perton, ‘ Quarterly Journal of the Geological Society,’ March
1871, p. 261, fig. 1. f Prof. Huxley, in ‘Medical Times and Gazette,’ 1857.
Eig. 16.
43
ment of the budding pleon of a Limulus at the stage figured by Packard (pi. y. fig. 27,
op. cit.) were so different in its vertical relation from that of the antecedent segment as
to support the assertion as to the limited locality of its attachment, the legitimate infer-
ence would he that it represented a corresponding part of a body-segment. If succes-
sively developed axial divisions of the body-segments are only to be regarded as such
when they happen to bear true appendages, many of the segments of the Mero-
stomes, besides the terminal one, must he relegated to the category of £ peculiar ’
median appendages — a view which would much obscure and complicate the problem of
determining the affinities of those primeval crustaceans on the basis of well-founded
homologies.
Concurring with my colleague, Mr. H. Woodward, in the views of the affinities which
are expressed by his extended application of Dana’s term Merostomata, which thus be-
comes something more than a mere synonym of Gronovan’s Xipliosura, I would remark,
in reference to the relations, in time, of the latter to Pterygotus, Lurypterus, and allied
extinct Silurian forms, that these manifest a more generalized character than do the
Xiphosures. One cannot say that they are persistent or arrested embryonal forms or
stages of development ; for we have seen that Limulus, as soon as the germ-heaps are
aggregated into unity or shape, assumes its concentrated character. Both families,
together with the Trilobitidce, exemplify that lower condition of the Crustacea which has
been expressed by the term Entomostraca, in which, as Mr. Woodward has well remarked,
the older, long and slender forms are analogous, in shape as well as in geological rela-
tions, to the macrurous Malacostraca, and the short and broad forms to the JBra-
chyura. If we further indulged in suggesting that the Merostomata might be the
ancestors of Arachnida, we might also conjecture that the Myriopods have come out of
Trilobites ; but this, at present, is not science. A superficial resemblance to the latter,
as we have seen, is shown by the absence of the pleon in the earlier stages of the King-
crab; but the very fact of the late appearance of this terminal division of the body,
after all the segments, with their appendages, of the antecedent division (‘thoracetron ’)
have been formed, is decisive against any real or representative resemblance of the
embryo Limulus to the Trilobites — on the acceptance, at least, of the original and valuable
observations by Barrande*, of the successive and later appearance of the abdominal
(‘ thoracetral ’) segments, in the space between the head (c cephaletron ’) and pygidium
(‘ pleon ’), in the embryos of Sao hirsutus,Agnostus nudus, and Trinucleus ornatus (fig. 12).
These developmental phenomena bear a significant analogy to those observed by New-
port f in the Julidce — the successive appearance, viz., of body-segments, in the space
(ib. b) anterior to the terminal or pygidial division (ib. c) ; such thoracetral segments
also appearing at successive moults, as in the Trilobites.
These, with other facts noted in the anatomical sections of the present paper, such as
the fusion of the pair of cephalic ganglia, the shortness and thickness of the ‘ crura ‘
* Systeme Silurien du Centre de la Boheme, 4to, 1852 ; section vii. pp. 257-276, “ Metamorphoses et mode d’ exist-
ence des Trilobites.”
f “ On the Organs of Reproduction and the Development of the Myriapoda,” Phil. Trans, vol. cxxxi. 1841 ; and
Owen, ‘ Lectures on Invertebrata,’ 8vo, 1855, p. 394.
G 2
44
connecting these with the suhcesophageal mass, in Limulus, giving the condition of that
part of the nervous system, as in Scorpio and Julus, as an ‘ annular centre,’ the nerve-
supply in Julus of two pairs of jointed appendages from the superaesophageal lohes
(Plate II, fig. 6), might be viewed in the following relation, — viz. that herein Limulus
manifested the more ‘ generalized type ’ of articulate structure, in which not only Aracli-
nidan but Myriapodal characters were associated with Crustaceous ones. But, in the
development of Limulus , the pleon or tail-spine (=pygidium) was the last to appear,
and, at its first budding, looked like a ninth segment of the thoracetron. Packard, as
we have seen, speaks of indications therein (transitory, indeed) of segmentation of the
crust ; and such indications I have shown to be more strongly and lastingly given by
the nervous system.
After formifaction and the attractive and repellant forces have produced, in the germ-
mass, the phenomena of segmentation and vegetative repetition (as manifested in the
similar and parallel heaps of granules, like bricks for the building), the inherited influences
seem to overrule the polaric ones and operate in differentiating and adaptive lines,
speedily showing the embryo-form of a Limulus ; which, like that of Astacus fluviatilis ,
F alamort adspersus, Crangon maculosus, JEriphia spinifrons , Spiders, and, one may add,
Cephalopods, goes straight to the goal of parental characters. There is no divergence
to a larval form enjoying for a term an active independent life. There is no metamor-
phosis, either nauplial, zoeal, or trilobitic.
Other representative analogies, however, can be adduced, which
are plain and intelligible. Arrest the development of Limulus at
the tailless stage (figs. 7, 8), and one gets a ‘ Belinumts or a
Frestwichia stadium’ (fig. 17). Stay awhile in serving the
warrant, and you have the short-tailed palaeozoic Limuloids — a
‘ Uemiaspis stadium.’
Segments indicated by the nerve-pairs but concealed or sup-
pressed by the crust at the base of the tail-spine in Limulus, were
Prestwichia rotundata. realized in Hemiaspis limuloides (H. Wd.). The progress from
the general to the special, from vegetative repetition to concentrative unity, is exemplified
in the living representative of the old Xiphosure (fig. 18) discovered by Salter in the
Fig. 18.
bed of a Silurian sea now contributing to form the county of Shropshire.
The ancestral pleon (c, ib.) has been almost “ rubbed out ” in the thousand-
fold generations of which the Salem King-crab is the heir ; but the palae-
ozoic taint sticks to the nerve-element. Or shall we say that Limulus,
made perfect for its sphere and habits of life, must have its “ alpen-stock ”
unbroken, of compact stuff without joints near the grasped end? But
then the teleologist or thaumatogenist has to give an account of the in-
termediate or ‘ evolutionary transitional ’ condition of the three pleonal
segments manifest outwardly, as doubtless by their nerve-pairs and pro-
bably ganglion-centres within, but soldered together or “ anchylosed,” in
Konig’s and Baily’s Belinurus, as in Limulus.
Should any persevere in objecting to the King-crabs’ being called Crustacea , by
Hemiaspis limu-
loides, Wd.
op. cit.
45
others the objection may he stronger to call them Arachnida or Myriapoda. Cha-
racters common to Limulus, with allied extinct gill-bearing, well-limbed Articulata,
have not a class-value. I believe myself at one with the best Carcinologists in refusing
to raise the Merostomata to an equivalency with Crustacea , i. e. to run them parallel
with and alongside of the rest of the branchiated Condylopods. A class, after all, is an
artificial group, a help to the classifier. One may call Limulus a Crustacean, and yet
discern in its anatomy the evidence of its more generalized structure as compared with
the Malacostraca. The merostomatous type preceded that of either the macrourous or
hrachyurous Crustacea ; and in Limulus , the sole living representative, we have been
able to detect characters subsequently overriding the crustaceous one, and intensified in
the air-breathing members of the Apterous Insecta of Linnseus.
As compared with its longer-bodied and many-segmented predecessors, Limulus itself
shows a concentrative specialization ; but vegetative repetition still reigns in the limb-
series. c Internal antennules,’ e external antennae,’ ‘ mandibles,’ £ maxillae,’ ‘ maxillipeds,’
e legs,’ — all work together by their spinigerous haunch-joints in subserviency to mastica-
tion, and all terminate in chelae. As compared with modern crabs, no structure is more
striking and significant than the resistence, so to speak, of the heart in Limulus to the
concentrative tendencies ; it is still the dorsal vessel, though the body-part containing it
has the breadth and shortness of the carapace of the crab, in which the heart is shaped
to match. In both Merostome and Brachyure the neural axis supplying the cephal-
etral limbs is annular : but, in modern crabs, the subcesophageal part is defined by
distance and by concomitantly elongated and slender, ‘ crura,’ or connecting tracts be-
tween it and the superoesophageal or cerebral part. This differentiation had not taken
place in Belinurus, Neolimulus, Prestwichia, and other palaeozoic predecessors of Bra-
chyura, whose organization we have to thank their longer-lived, lingering representative
genus for enabling us to peer into.
That such glimpses, with concomitant tracing of the development of the individual
Limulus, afford us some ground, and that the like work, with persevering quest of its
palaeozoic fossil allies, may afford more ground for at least guessing at the ways in which
a preordained plan of derivation by congenital departures from parental form has
operated in originating the various branches from a common ancestral articulate stem,
is an encouraging faith.
That old Ocean should have afforded the chance conditions of origin of crustaceous
subclasses, orders, genera, species, by ‘ Natural Selection,’ is not conceivable by me : the
metaphysical facts that there is { will,’ that a ‘ sense of the beautiful ’ exists, that e a love
of virtue ’ operates, opposes the supposition. Such facts suffice for the rejection of a
‘ Nature ’ working without will, taking no counsel of either the good or the beautiful,
casting up from her dark abyss only eternal transformations of herself, furthering, with
the same restless activity, decline and increase of organs, death and life of indivi-
duals, extinction and origination of species. Nevertheless I hold by the conviction that
all forms and grades of Articulata are due to ‘ secondary cause or law ’ as strongly as
46
when I expressed the same belief in regard to the Vertebrata *, and defined it as “ the
deep and pregnant principle in Philosophy ” + evolved in the researches on the general
homologies and archetype of the Vertebrate Skeleton.
The series of facts added to biology in the present century by these researches, with
other correlated series, palaeontological and embryological, of the kinds illustrated in the
present memoir, render “ thaumatogeny J”, or the hypothesis of direct creation of species,
inadequate to their explanation. The invocation, by Cuvier, of successive miraculous
interventions, creative and destructive, to solve or explain the phenomena of the succes-
sion of extinct species, chiefly made known by his grand discoveries, recalls the com-
plexity of cycles and epicycles invoked to explain the facts of astronomy as they had
accumulated in the time of Copernicus. He knew not how the twofold movements of
the earth (rotation on its axis, revolution about the sun) were governed ; but the hypo-
thesis on those postulates simplified the comprehension and explanation of the pheno-
mena of the heavens.
So the way of operation of “ nomogeny,” or the incoming of species by secondary cause,
remains to be demonstrated ; but its expository relation to the phenomena is a guarantee
of its truth. Volition, with exercise and disuse of parts, invoked by Lamarck, are of the
nature of “causse verse,” but inadequate; premature births, congenital departures from
parental characters, suggested by “Vestiges,” meet some of the phenomena; “Natural
Selection ” suggests other conditions of “ nomogeny.”
Biology is in its Copernican stage. The analogy to Astronomy is close. The objection
to the Canon of Frauenberg, that the rate of the whirl required by his hypothesis would
send into space all things loose on the earth’s surface, is akin to some of the cavils, as
seemingly fatal, to the evolutional view. The analogy of the course of physical science,
and the accelerated rate of progress of those of life, since DeMaillet, Lamarck, and
Oken, ceased to be exceptional advocates of Nomogeny, justify the expectation that this
hypothesis will be developed into a known law, and one day receive its crowning demon-
stration from the Newton of Biology.
§ 12. DESCRIPTION OF THE PLATES.
Plate I.
Limulus polyphemus.
Fig. 1. Alimentary canal, hepatic ducts, part of the fiver, and of the muscular system, in situ , viewed
above, or from the dorsal aspect.
a. Cephaletron : a" its postlateral spinous pro-
duction (f genal spine * of Trilobitology) .
b. Thoracetron.
c. Base of pleon.
i. Compound eye.
a 1. Ocelli.
h. Entapophysial pit of hindmost (by anchy-
losis) cephaletral segment.
* On the Nature of Limbs, 8vo, 1849, p. 86.
J Anatomy of Vertebrates, vol. iii. p. 814.
t lb. p. 10.
47
i. Intestine.
i 1-4. Entapophysial pits of thoracetron.
k. Terminal spine of the ocular or medilateral
ridge of the carapace.
l. Anterior hepatic duct.
m. Posterior hepatic duct.
m 1-6. Marginal articulated spines of thorace-
tron.
Fig. 2. Heart and vessels, with parts of ovaria, in situ,
a, b, c, as in fig. 1.
a. Heart.
b. Part of pericardial sinus.
b' . Part of pericardial sinus laid open.
c. c. Ostia venosa.
e, e. Neural arteries.
f, f. Epimeral arteries of cephaletron.
g, g' . Epimeral arteries of thoracetron.
h, Ocellar artery.
n 1-7. Marginal spinous angles of thoracetral
segments.
n. Liver.
n" . Hepatic lobe, partially injected from the
duct.
o. Dilated beginning of intestine.
s, Stomach.
t, t. Fasciculi of f depressores thoracetri’ muscles .
viewed from above.
i 3—6. Entapophysial pits of thoracetron.
n. Medilateral vein and sinus of cephaletron.
o. Oviduct.
p. Anterior thoracetral vein.
q. Middle thoracetral vein.
r. Posterior thoracetral vein.
s. Median thoracetral vein.
u, u. Fasciculi of the f levatores telsoni/
Plate II.
Fig. 1. Longitudinal section, with side view of the nervous system, the heart laid open, and the alimentary
canal in situ. The explanation of the parts is given in the outline of this figure in Plate II a.
2. Longitudinal section of cephaletron and fore part of the alimentary canal, showing the relations
to the mouth of the cephaletral limbs. The explanation of the parts is given in the outline of
this figure in Plate II a, which also includes figures 3, 4, 5.
6. Anterior view of the head of Julus terrestris, dissected to show the cephalic or superoesophageal
lobes, and the nerves to the eyes, antennae, and mandibles (magn. 6 diameters) .
7. Upper view of the brain and nerves, with the beginning of the ventro-chordal part of the neural
axis of Julus terrestris (magn. 6 diameters).
8. Upper view of the cephalic lobes, with the optic nerves and disposition of the beginning of the
stomato-gastric system of nerves upon the corresponding part of the alimentary canal of Julus
terrestris (magn. 6 diameters).
Plate II a.
Fig. 1. Outline of figure 1, Plate II., giving a side view of heart, alimentary canal, and nervous system.
a, b, c, i & a 1, as in PI. I. m. Antenna.
a'. Under or vaulted surface of cephaletron. iv. Mandibula.
b". Anal segment of thoracetron. v. Prsemaxilla.
n. Antennula. vi. Maxilla.
48
vii. Maxilliped.
* Chilarion.
viii. Lid-plate.
ix. First gill-plate.
x. Second gill-plate.
xi. Third gill-plate.
xii. Fourth gill-plate.
xiii. Fifth gill-plate.
b. Dorsal wall of cephaletron.
c. Ventral wall of cephaletron.
d. Digging-border of cephaletron.
e. Holding-border of cephaletron.
g. Entering process of thoracetron.
h. Entosternon.
i. Intestine.
m 1 . Levatores thoracetri.
m 7. Levatores telsi.
m 8. Obliqui telsi.
m 9. Depressores telsi.
n a. Ocellar nerve.
n a. Ocular nerve.
n ii. Antennular nerve.
n hi. Antennal nerve.
n iv. Mandibular nerve,
n v. Premaxillary nerve.
n vi. Maxillary nerve.
» vii. Maxillipedal nerve.
» viii. Opercular nerve.
n *. Chilarian nerve.
n ix-xiii. Nerves to branchigerous limbs.
n xiv. Nerve of anal segment.
o. Ostia venosa.
pi. Pleonic plexus.
r. Muscular wall of heart.
s. Neural artery.
t. Pleonic artery.
a. Brain, or superoesophageal nervous centre.
/3. Neural ring.
y. Neural cords.
8. First ganglion.
e. Second ganglion.
£. Third ganglion.
7j. Fourth ganglion.
0. Fifth or terminal ganglion.
X- Ganglionic loop.
X. Pleonal nerve, or continuation of neural cord.
p. Principal nerve of
pi. Pleonal plexus.
a 1-4. Dorsal nerves of four anterior coalesced
pleonal segments.
Fig. 2. Outline of figure 2, Plate II, giving a vertical longitudinal section of fore part of cephaletron and
alimentary canal, with the cephaletral limbs or appendages of the left side. (The Preparation
is No. 477 a, in the Physiological Series of the Museum of the Royal College of Surgeons :
c Descriptive and Illustrated Catalogue/ 2nd ed. 8vo, 1852, p. 132.)
a. ii to vii, and *, as in figure 1.
b. Dorsal wall of cephaletron.
c. Ventral wall of cephaletron.
d. Digging-border.
/. Chi tine.
g. Pigment-layer.
ce. (Esophagus.
h. Entosternon.
i. Intestine.
1. Anterior hepatic duct.
m. Posterior hepatic duct.
p. p. Carding-plates, or palpi, of haunch -joints
of III-VII.
*. Chilarion, a seemingly serial repetition of
haunch-joints.
q. Median lobe of ovary.
r. Pro ventricular or cardiac part of stomach.
s. Gizzard.
mt. Pyloric prominence.
Fig. 3. Outline of the maxilla (6th cephaletral limb).
1. Coxa. 4. Cnemion.
p. Its “ palp,” or carding-plate. 5. Propes.
2. Basis. 6. Dactylus.
3. Merion.
49
Fig. 4. Outline of maxilliped (7th cephaletral limb).
1-5, as in fig. 3. ' r. Flagellum.
6. Lamelligerous joint. s. Cnemial appendage.
7. Chelate extremity. t. Terminal lamellae.
Fig. 5. Mandibula (4th cephaletral limb) dissected for the muscles.
a. Extensor basis mandibulae. e. Propedal entapophysis.
b. Flexor basis. /. Flexor propedis.
c. Merional entapophysis. g. Flexor dactyli.
d. Flexor merioni cnemiique.
Plate III.
Limulus polyphemus.
Right half of the male King-crab, with the nervous system dissected from the ventral surface. The
explanation of the parts in this figure is given in the outline copy in Plate IV.
Plate IV.
Fig. 1. Outline of figure 1 in Plate III., showing the nervous system, from the lower or ventral aspect.
a'. Inferior concave surface of^cephaletron. n xv. Nerve of anal segment.
b\ Infero-lateral surface; b" Infero-median sur- (The other letters and figures
as in Plate II a.)
face of thoracetron.
Fig. 2, Compound eye and terminations of ocular nerve, from the dorsal aspect : magnified 6 diameters
[Limulus polyphemus ) .
d. Dorsal branch and divisions of ocular nerve. v. Yentral branch and divisions of ocular nerve.
Fig. 3. Compound eye of a Trilobite [Phacops conophthalmus ) : magnified 3 diameters.
Fig. 4. Portion of the same compound eye : magnified 10 diameters.
Fig. 5. Entosternon and attached parts of muscles. (After Yan der Hoeven, op. cit.).
Fig. 6. Chief parts of ovarium, oviducts, and opercular
polyphemus ) .
wi". Muscles of operculum.
o. o. Oviducts.
p. Oviducal outlets.
q. Postero-median lobe of ovary.
q*. Antero-median lobe of ovary.
q" . Antero-lateral loops and branches.
plate, from the upper or inner surface [Limulus
* q **. Postero-lateral cephaletral branches,
r. Articular surface of opercular plate.
2. Second joint of opercular plate.
3. Third joint of opercular plate.
4. Appendages of opercular plate.
Fig. 7. Section of terminal part of oviducts. (After Yan der Hoeven, op. cit.)
a. Oviduct; b, anterior labium ; c, portion of opercular plate.
Fig. 8. Portion of opercular plate with terminations of the sperm-ducts, p. (After V an der Hoeven, op. cit.)
Plate Y.
Limulus polyphemus.
Fi£T. 1- Nervous system, from the upper or dorsal aspect.
a. Superoesopliageal mass. b. (Esophageal ring
50
n a. Ocellar nerve.
n a. Ocular nerve.
n 3. Gastric nerve.
n 4. 1st Epimeral nerve.
n 5. 2nd Epimeral nerve.
n 6. Its recurrent branch.
n 7. 3rd Epimeral nerve.
n 8. 4th Epimeral nerve.
n 9. 5th Epimeral nerve.
n 10. 6th Epimeral nerve.
nil. 7th Epimeral nerve.
n 12. 8th Epimeral nerve.
n 13. 9th Epimeral nerve.
n 14. 10th Epimeral nerve.
n 15. 11th Epimeral nerve.
n 16. 12th Epimeral nerve.
n 17. 13th Epimeral nerve.
n 18. 14th Epimeral nerve.
n 19. 15th Epimeral nerve.
pi. Pleonal plexus.
(The preparation is No. 1303 c, in the Physiological Series of the Museum of the Royal College
of Surgeons.)
Fig. 2. Left gill-limb, with attached branchia.
a. Tract of efferent branchial vessels.
Fig. 3. Branchial lamella ; a, efferent plexus.
All the figures, save where otherwise stated, are of the natural size.
Woodcuts.
Fig. 1. Cell-egg, with yolk-cells developing: magnified 130 diameters (Limulus polyphemus).
Fig. 2. Impregnated egg after disappearance of primitive cells : magnified 30 diameters : id.
Fig. 3. Section of egg-coat and germ-trace : magnified 130 diameters : id.
Fig. 4. Ovum with embryo-trace : magnified 10 diameters : id.
Fig. 5. Ovum with embryo further developed, side view: magnified 10 diameters: id.
Fig. 6. Ovum with embryo, under-view : magnified 10 diameters : id.
Fig. 7. Embryo Limulus, just before hatching, back view : magnified 13 diameters.
Fig. 8. Newly hatched Limulus, under-view : magnified 9 diameters.
Fig. 9. Nauplius, or young of Apus cancriformis : magnified.
Fig. 10. Nauplius, or young of Cyclops quadricornis : magnified.
Fig. 11. Zoea, or larva of Penceus.
Fig. 12. Five stages of the development of a Trilobite ( Trinucleus ornatus, Stb.), selected from the more
numerous series in Barrande’s ‘ Systerfle Silurien du Centre de la Boheme/ 4to, 1852, pi. 30.
a. Larva consisting of the cephaletron a, and pleon c : b. Larva showing the characteristic elon-
gation of the posterior angles of the cephaletron, or “ genal spines : ” c. Larva with one thorae-
etral segment b : d. Larva with two thoracetral segments b ; e. Young animal showing mature
number of six thoracetral segments b, and fully lengthened “ genal spines.”
Fig. 13. Eurypterus Scouleri, Hbt. ; reduced restoration (Carboniferous Limestone) .
Fig. 14. Pterygotus anglicus, Ag. ; reduced restoration (Devonian of Forfarshire).
Fig. 15. Slimonia acuminata, II. Wd. ; reduced restoration (Upper Silurian).
Fig. 16. Styloneurus Logani, H. Wd. ; reduced restoration (Upper Silurian).
Fig. 17. Prestivichia rotundata, H. Wd. ; two-thirds nat. size (Coal-measures).
Fig. 18. Hemiaspis limuloides, H. Wd. ; two- thirds nat. size (Lower Ludlow).
(The figures 12-18 are copied from those reproduced by H. Woodward, in his excellent paper
“ On the relationship of the Xiphosura to the Eurypterida and to the Trilobita and Arach-
nida” in the ‘Popular Science Review ’ for October 1872.)
Plate. I.
IIS- Ef ‘ I I Fig. 2.
Plate II.
Limulus polyphemus.
1 . Longitudinal section, with side view of the nervous system, the heart laid open, and the alimentary
canal in situ. The explanation of the parts is given in the outline of this figure in Plate II a.
2. Longitudinal section of ceplialetron and fore part of the alimentary canal, showing the relations
to the mouth of the cephaletral limbs. The explanation of the parts is given in the outline of
this figure in Plate II a, which also includes figures 3, 4, 5.
G. Anterior view of the head of Julus terrestris, dissected to show the cephalic or super oesophageal
lobes, and the nerves to the eyes, antenna?, and mandibles (magn. 6 diameters) .
7. Upper view of the brain and nerves, with the beginning of the ventro-chordal part of the neural
axis of Julus terrestris (magn. 6 diameters).
8. Upper view of the cephalic lobes, with the optic nerves aud disposition of the beginning of the
stomato-gastric system of nerves upon the corresponding part of the alimentary canal of Julus
terrestris (magn. 6 diameters).
1
A.T.Hollrck ckrcmo-Jitk.
MmlernBros chroono -jiup .
Plate II ^
A.T.tLoHick chromo AlOn..
Plate IV.
Fig. 1.
Plate V.
Pig. 1.
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